THE UPPER TRIASSIC OF WESTERN TEXAS. 55 



A review of the known skulls of the Parasuchia reveals the fact that they may be 

 divided into two more or less clearly marked groups — those with long, slender, depressed 

 rostra and those with elevated rostra. To the first group, the Mystriosuchid group, 

 belong Mystriosuchus, Rhylidiodon, Paleorhinus, and Promystriosuchus; to the second 

 group belongs Phytosaurus (Belodon). Intermediate between the two are the forms 

 which have been described by Cope as Belodon buceros and by Mehl as Machceroprosopus 

 validus. In the opinion of some workers, Belodon buceros Cope and Machoeroprosopus 

 validus Mehl should be placed in the genus Phytosaurus. The two specimens described 

 in this volume as Leptosuchus help to further bridge over the gap between the two groups. 



The form described as Promystriosuchus belongs in the Mystriosuchid group and 

 is nearest to Angistorhinus. Because of its retention of certain primitive characters, 

 it will be best to compare it first with Mesorhinus, the earliest and most primitive of the 

 Parasuchia. Mesorhinus was described by Jaekel 1 from the middle Bunter sandstone 

 of Bernburg. 



Comparison with Mesorhinus. 



The anterior portion of the rostrum of the specimen was not recovered, and 

 Huene considers that it has been made too long in the restoration by Jaekel. Granting 

 the correctness of Jaekel's restoration, which would be the maximum possible length 

 of the rostrum, there is still a great disparity between the relative lengths of the prenarial 

 and postnarial portions of the skull and the same measurements in Mystriosuchids 

 from the Upper Triassic Measuring from the anterior end of the narial opening, Jaekel's 

 figure (as copied by Huene) gives a relative proportion of lengths between the prenarial 

 and postnarial portions of the skull as 5 : 6; the same measurements on the figure given 

 by Abel (St annuo der Wirbelthiere, p. 516) give proportions of 4 : 5. In Promystriosuchus 

 the rostrum has become relatively much elongated, a distinctive character of the special- 

 ized Parasuchians of the Upper Triassic, the proportional lengths being as 3 : 2. In 

 Mesorhinus the narial opening is elongate and clearly divided by a septum at the level 

 of the surface of the skull, formed by an anterior extension of the nasals; the antorbital 

 vacuity reaches either to the middle of the narial opening or only a little way anterior 

 td the posterior edge, according to figures of the lateral and the upper view of the skull 

 as given by Jaekel (copied by Huene); the lateral temporal opening is nearly circular; 

 the parieto-squaniosal bar defining the inner border of the supratemporal foramen is 

 complete and at the level of the roof of the skull; the supratemporal foramen itself is 

 entirely on the top of the skull; the squamosals do not extend far posterior to the occip- 

 ital condyle. In Promystriosuchus the narial opening is round, is surrounded by a high 

 rim, and the septum is far down within the cavity of the nares; the antorbital opening is 

 elongate and reaches as far forward as the middle of the narial opening; the lateral tem- 

 poral opening has assumed, more or less perfectly, the characteristic parallelogrammic 

 form of the Parasuchia; the parieto-squaniosal bar is at the level of the roof of the skull, 

 and the supratemporal opening is entirely upon the upper surface; the squamosals do 

 not extend far posterior to the occipital condyle. 



On the posterior surface of the skull the shortness of the opisthotic process in 

 Mesorhinus is a striking difference from the condition found in the younger Parasuchia. 



On the lower surface the palate of Mesorhinus, as figured by Jaekel, is radically 

 different from that of the majority of the Parasuchia. The pterygoids lack the strong 

 cxt final process; the transverse articulates with the outer edge of the pterygoid instead 

 of underlying an external process; the palatine vacuity is large, lying between the pala- 

 tines, pterygoids, maxillaries, and transverse. The peculiar articulation between the 



'.Jaekel, ().. Sitz. Her. d. Cesell. N'.iturf. I-'reuwle. Berlin, No. 5, p. 197, 1910; see also Huene, F. v., Geol. u. 

 Paleont Abhdlg., X. K, Bd. x, lift. I, s. 50, 1911. 



