222 PROF. H. Gt. SEELEY ON AEISTOSTJCHTJS PUSILLTJS. 



it is manifest, I submit, that the Poihilopleuron pusillus and Mega- 

 losaurus Buchlandi belong to two dissimilar genera. But the 

 attempted affiliation of the Wealden fossil, now under discussion, 

 to the genus Poihilopleuron does not establish the sacral characters 

 of the genus Poihilopleuron, or furnish any ground for associating 

 the genus with the Crocodilia. The characters assigned to this 

 fossil would rather go to show that it belongs to a genus which can 

 have no near affinity to Poihilopleuron. In his Report on British 

 Fossil Reptiles, Sir R. Owen fully described the characters of the 

 sacrum of Megalosaurus, and he there points out that the neural 

 arch is shifted in position, so that it overlaps the centrums of two 

 contiguous vertebras, as in the Ostrich and other birds and some 

 Chelonians, so as to cause the perforation for the sacral nerve to be 

 placed above the middle of the centrum, and that the sacral ribs 

 are given off transversely at the junction of the bodies of the ver- 

 tebras. When the fossil named Poihilopleuron pusillus is examined, 

 both these conditions of the Dinosaurian sacrum are found to be 

 wanting. Each sacral centrum supports its own neural arch, the 

 neural foramen has the same relative position as in other parts of 

 the vertebral column ; and the sacral ribs are given off from the 

 bodies of the vertebras, and not from the suture between them. It 

 is therefore evident that the fossil is far removed from Megalosaurus, 

 and inferentially from Poihilopleuron. As it differs in fundamental 

 characters from known Dinosaurs, while there are strong reasons 

 for believing it to be Dinosaurian, I regard it as the type of a new 

 genus, Aristostichus, 



Sir R. Owen has figured some bones which were associated with 

 this sacrum (see PI. XII. fig. 14). There is a median symmetrical 

 bone, to which are attached portions of a pair of rib-like bones, on 

 which the author observes, " the nearest guess I can make as to 

 their nature is that they represent part of the series of abdominal ribs 

 with their sternum." These remains I regard as the pubes. 



An ungual phalanx is figured on the same plate, and briefly 

 described as being " of the rapacious type." This fossil I think 

 should probably be rejected, as not being a portion of the same 

 animal, for it shows all the characters of a claw-phalange from the 

 fore limb of an Ornithosaur, though it may be observed that in 

 Loslaps the claw-phalanges are as much compressed from side to 

 side. With these differences in the interpretation of the remains a 

 new description of their characters becomes a necessity. 



I will first examine the pubic bones and assume, as Sir R. Owen 

 has done, that these bones are in natural association with the 

 sacrum, with which they are still closely connected by matrix, 

 though they are displaced and are twisted round, so that the anterior 

 border is directed posteriorly. 



The pubic bone is imperfect proximally, and the expanded portion 

 which united with the ischium and ilium is lost. The parts pre- 

 served are the anteriorly directed, distally extended, rod-like parts 

 of the pubes, which converge towards their distal extremities, where 

 they merge in a horizontal posterior extension capable of assisting 



