220 



PACHYRHIZODUS, Agassiz. 



The genus as seen in our fossils is defined as follows : Muzzle flat; pre- 

 niaxillaiy bones rather long, with two larger teeth together near the anterior 

 end behind the usual external series; maxillary and mandibles with single 

 series of simply cylindric subequal curved teeth ; mandibular rami closely 

 articulated by a ligament. The teeth possess short stout fangs, occupying 

 alveoli, of which the inner side and part of the anterior and posterior walls 

 are incomplete. The teeth are, in fact, more or less pleurodont, but the 

 extremity of the root is received into the conic fundus of the alveolus. They 

 boar a superficial resemblance to those of a mosasauroid genus. Their mode 

 of succession appears to be as follows: The crown of the young tooth was 

 developed in a capsule at the base of the crown, or on the inner side of the 

 apex of the thick root. The absorption which followed excavated both the 

 former and the latter ; but the crown was evidently first shed. Finally, the 

 old root disappeared, and the new one occupied the alveolus, leaving a free 

 separation all round. Finally, on the accomplishment of the full growth of 

 the root, it became anchylosed to the alveolus all round. The pleurodont 

 position of the tooth facilitated the shedding of the root very materially. 



The premaxillary bones are well developed, but the maxillaries are more 

 so, and enter largely into the composition of the border of the mouth. There 

 is a well-developed angle of the mandible, but no coronoid bone. 



Well-preserved fragments of a large specimen of P. latimentum include 

 a number of bones whose relations are not readily determined. A subrhom- 

 bic element, with a sutural edge turned ? upward, and the fupper surface 

 sculptured with ridges, has a small patch of teeth en brosse on the 1 lower 

 face. An elongate bone, with gently concave longer sides and crenate edges, 

 is very dense; perhaps a preorbifal. A large flat parallelogrammic bone, with 

 a narrow articular surface at both ends, the one shorter than the other, with 

 thin lateral borders, is possibly interopercular. The hyomandibular has its 

 pterygoid articular border forming an obtuse angle with that for the inferior 

 quadrate, and the latter a right angle with the line of junction with the oper- 

 culum. Between the latter is a concave l)order marking half a circle. If the 

 relations of this piece be jnoperly determined, there is no peduncle for the 

 operculum. 



