EXTINCT CROCODILIA. 381 



till after the Jurassic period that the majority are found in 

 association with fresh-water and land forms. The living forms 

 are inhabitants of fresh water, a few species extending into 

 estuaries. 



Huxley in his classical memoir on the " Crocodilia of the Elgin Sand- 

 stones" (loc. tit.) divided the Crocodilia into three groups which form a 

 series in respect of certain osteological characters, viz. the structure of the 

 palate, the condition of the eustachian passages, the form of the vertebrae, 

 and one or two other points. These groups he named ( 1 ) the Parasuchia 

 in which the palatines and pterygoids have no palatal plates (Fig. 203), 

 the eustachian passages are unenclosed by bone and the vertebrae are 

 amphicoelous ; (2) the Mesosuchia with amphicoelous vertebrae, eusta- 

 chian passages partly enclosed in bone, and palatal plates to the palatines 

 but not to the pterygoids (Fig. 204) ; (3) the Eusuchia with procoelous 

 vertebrae, eustachian passages completely embedded in bone, and palatal 

 plates to both pterygoids and palatines (Fig. 205). He also showed that 

 these groups succeed one another in time in a manner which is consistent 

 with regarding them as having being derived successively from one another, 

 i.e. the Mesosuchia from the Parasuchia, and the Eusuchia from the Meso- 

 suchia. In short he showed that the Parasuchia appear in and are con- 

 fined to the Trias, the Mesosuchia appear in the Upper Lias and are con- 

 tinued to the Wealden, while the Eusuchia extend from the Later Cre- 

 taceous to the present time. To use Huxley's words : " The order of 

 occurrence of the three divisions of the Crocodilia in time coincides with the 

 order in which they depart from the lacertilian type and put on special 

 crocodilian characters ; and this palaeontological fact is in precise accord- 

 ance with the needs of the theory of evolution." 



In the following systematic treatment of the group, this division of 

 Huxley has been partly followed, but the Mesosuchia, which more closely 

 resemble the Eusuchia than they do the Parasuchia, have been united with 

 the Eusuchia, and a new group the Pseudosuchia has been established for 

 certain imperfectly known forms discovered since the date of Huxley's 

 memoir. 



Order 1. PARASUCHIA.* 



Extinct Crocodilia of considerable size, confined to the Triassic forma- 

 tion (Keuper), Europe, Asia, and N. America. With two longitudinal 

 rows of dorsal scutes, and more numerous rows of ventral scutes (composed 

 of one piece only), biconcave vertebrae, long premaxillae, external nostrils 

 placed far back near the upward directed orbits, internal nares at the 

 front end of the palatines ( Fig. 203). The palatines and pterygoids do not 

 meet in the middle line, and the eustachian passages are not enclosed by 

 bone. A postorbital behind the orbit. Parietals and frontals paired. 

 Acetabulum formed of ilium, ischium and pubis. A clavicle is present. 

 In the separate, posteriorly placed nostrils, in the large preorbital fossae, 

 in the form of the pterygoids and basiphenoid they resemble the Dino- 

 saurs. In some other characters (e.g. the separate postorbitals, the 

 paired parietals and frontals, the palate, the well developed ventral ribs, 

 and the clavicle) they approach the Rhynchocephalia. Belodon v. Meyer, 

 Stagonolepis Ag., Parasuchus, Huxl. 



* Huxley, Quart. J. Geol. Soc., 1859, 15, p. 440, and 1875, 31, p. 423. 



