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502 OSBORN, THE REPTILIAN SUBCLASSES DIAPSIDA AND SYNAPSIDA. In Rhynchocephalia the ribs, primarily two-headed in embryogeny, become secondarily single-headed by the degeneration of the hypocentral articulation of the head and the development of the tubercular attachment to the pleurocentra. The sacral ribs, 1-3 in number, single-headed, are opposite the pleurocentra. The caudal ribs vary from 6 in Palcohatleria to i i in Stereoslernum. ABDOMINAL RIBS. The principal facts regarding abdominal ribs are summarized on page 474. The presence of stout median and lateral series is highly characteristic of the Diaptosauria. GIRDLES. The primitive constitution of the shoulder girdle includes a large procoracoid suturally separate from the coracoid (Dime/rodon, Hyperodapedon, Procolopihon); the secondary condition, in which these elements are reduced and the procoracoid united or wanting, is seen in the Protorosauria, Proganosauria, Choristodera, certain Rhynchosauria (Rhynchosaurus), and Rhynchocephalia. The primitive interclavicle expands into a rhombic anterior plate (Pro/orosaurus, Palafohat/eria), suggesting that of Stegocephala; secondarily it becomes T-shaped in Pelycosauria, Hyperodapedon, Proganosauria, Choristodera, and Rhynchocephalia. The primitive pelvic girdle similarly consists of plate-like pubes and ischia, as in the Stegocephala; puboischiadic fenestra small or wanting (Protorosauria, Rhynchosauria, Procolophonia, Choristodera); pubis perforated by an obturator foramen. Secondarily the puboischiadic or thyroid fenestra develops separating the pubis and ischium ventrally (certain Pelycosauria, all Rhynchocephalia), the obturator foramen frequently becoming confluent with the puboischiadic fenestra. The primitive ilium is probably narrow and rod-like (Rhynchocephalia, Proganosauria). In cursorial and ambulatory forms (Protorosaurus, Paloohat/eria) the ilium is secondarily expanded, with an arched superior border, as in Dinosaurs. In Pelycosauria and Choristodera the dorsal expansion is asymmetrical, suggesting that of the Crocodilia. LIMBS. The primitive limbs are of a short ambulatory type (Procolophon, Rhynchosaurus) with distal phalanges curved, indicating the presence of more or less pointed, prehensile claws. In aquatic forms the terminal phalanges and claws degenerate. Elongation for cursorial habit, especially in the hind limbs (Protorosauria), and abbreviation for aquatic habit, is secondary. Humerus.-A prominent deltoid crest is secondarily developed in Pro/orosaurus. The distal or ' condylar foramina' of the humerus exhibit no fixed character and it would appear that the value of this character in phylogeny and
OSBORN, THE REPTILIAN SUBCLASSES DIAPSIDA AND SYNAPSIDA. 503 classification has been overrated by Baur and others. 'Radiocondylar' or 'ectepicondylar' foramina are observed in all Protorosauria (Pro/orosaurus, Palceoha/leria, Kadaliosaurus). On the other hand, 'ulnocondylar' or 'entepicondylar' foramina develop in Proganosauria (Mesosaurus and Slereos/ernum), the Choristodera (Chamfisosaurus), and certain Rhynchocephalia (Homceosaurus). In other types both an ulnocondylar foramen and a radiocondylar foramen or groove are developed (certain Rhynchocephalia-Anguisaurus minor, Sphenodon-and certain Pelycosauria). With these striking variations of the humeral perforation in different orders and genera we cannot determine at present which is the most primitive foramen-whether the radiocondylar, the ulnocondylar, or the perforation of both condyles. The distal separation of the propodials (ulna and radius, tibia and fibula) by the intermedium, in certain Procolophonia, Proganosauria, and Rhynchocephalia is either primitive (as in Stegocephala) or a secondary adaptation; it is probably primitive. PODIALS. Cafrpus.-The primitive carpus undoubtedly includes ten elements, three proximal carpalia (radiale, intermedium, ulnare) equal in size, two centralia, five distal carpalia as in certain Protorosauria (Prolorosaurus). In the embryo Sphenodon there are said to be eleven to twelve elements, including a pisiform and possibly a third centrale (Howes). In the secondarily reduced carpus (e. g., Slereosternum) there are but one centrale and four distal carpalia. Intracarpal and intratarsal foramina are observed in Proganosauria and young Rhynchocephalia. Tarsus.-No ideally primitive tarsus with three distinct proximal elements (tibiale, intermedium, fibulare), free centrale or centralia, and five separate distal tarsalia, has yet been found among the Diaptosauria. More or less coalescence (astragalus = tibiale + intermedium + centrale) or degeneration of these elements has occurred in every known type. Five separate distal tarsalia are observed only in certain Protorosauria (Palcohaleria) and Proganosauria (Mesosaurus and Slereos/ernum),.a unique feature among rep/fies; in all other Diaptosauria distal tarsalia 4 and 5 are united. MANUS AND PES. Phalanges.-The primitive phalangeal formula in the manus is undoubtedly 2, 3, 4, 5, 3. This is observed in all forms except the otherwise primitive Procolophon, in which the phalangeal formula in the manus is 2, 3, 4, 5, 4. In the pes the phalangeal formula is apparently 2, 3, 4, 5, 3 in certain Protorosauria (Palatoha//eria), while in other Protorosauria (Pro/orosaurus) it is 2, 3, 4, 5, 4; the latter is the prevailing formula in the pes of Diaptosauria, as it is found in the Proganosauria, Rhynchosauria, Procolophonia, and Rhynchocephalia.
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MEMOI RS OF THE Se t r H isto-ry. V
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VOLUME I, PART VIII. THE REPTILIAN
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452 OSBORN, THE REPTILIAN SUBCLASSE
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Page 40 and 41: Fig. 17 a. Stereosternum tumidum Co
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Page 65 and 66: INDEX TO VOLUME I. [New names of ge
Page 67 and 68: Aphelops profectus, 358, 373 superc
Page 69 and 70: Hoplophoneus pr1mevus, 394 robustus
Page 71 and 72: Pleurosaurus, 463, 468, 494, 495, 4
Page 73 and 74: Sesia albicornis, 226, 229, 230, 27
Page 75: Trochilium tipuliformis, 286 tricin
Page 78: PUBLICATIONS OF THE American Museum
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