i66 



VERTEBRATE SKELETON 



basisphenoid. Recent species have a large transversum, (lacking in 



some extinct species) in front of which is a large palatal vacuity. 



Pterygoid and transversum give off a strong ventral process on which 



the mandible plays. 



A distinct ahsphenoid on either side closes the cranial cavity 



in front ; an orbitosphenoid is lacking. The large interorbital septum 



(cartilage or partially ossified) is supported 

 on the cartilage rostrum and continues as 

 the nasal septum. The quadrate is fixed 

 on the upper medial surface by the 

 squamosal, by the quadratojugal on the 

 antero-inferior side. The quadrate ex- 

 tends obliquely backwards and down- 

 wards, so that the tympanic cavity fies 

 above it. Teeth are confined to the 

 margins of the jaws and are thecodont in 

 all families. All of the bones of the lower 

 jaw, goniale excepted, retain their indi- 

 viduaHty, and there is a large vacuity on 

 both lateral and medial sides (fig. 177), and 

 in some genera the articulare is perforated 

 by a tube (siphoneum) leading to the 

 interior of the bone, and containing an air 

 passage coming from the quadrate. The 

 large, nearly quadrate, hyoid body is 

 cartilage (sometimes partly ossified) and 

 is concave above to accommodate the 

 larynx. It bears a pair of ossified cornua, 

 regarded as the first branchial arch; the 

 cornua are not connected with the cranium. 



Fig. 178.— Floor of skull of 

 Alligator, bo, basioccipital and 

 condyle; c, choanas; e, opening 

 of Eustachian tube; m, maxilla; 

 pi, palatine; pt, pterygoid; q, 

 quadrate; qj, quadratojugal; s, 

 zygomatic. 



PARASUCHIA (Phytosauria) (fig. 179) are related in skull structure to Rhyn- 

 chocephals, Crocodiles and Dinosaurs. The separate nares are far back on the 

 cranium and have vacuities beside them. Both temporal fossae occur, separated 

 by an arcade largely postorbital, the squamosal contributing but little. Parie- 

 tals and frontals are paired and some have a parietal foramen. The choanae 

 are just behind the palatal processes of the maxillae, palatines and pterygoids 

 not meeting in the middle line. Apparently Meckel's cartilage was largely 

 persistent, and there is a vacuity on the outer side of the lower jaw. The 

 animals were like crocodiles in form and general relations of cranial bones; 

 like Rhynchocephals in having postorbitals, paired frontals and parictals, and 



