146 



MK. A. S. WOODWARD ON SQTJATIl^A CEANEI Als^D 



examples of Belonostomus cinctus supply this deficiency in our know- 

 ledge of the genus to a considerable extent, and explain the peculi- 

 arities of the fragmentary fossils described by Agassiz, besides making 

 known more completely than before the largest presymphysial bone 

 recorded in the annals of vertebrate anatomy. 



The principal specimen is shown two thirds of the natural size 

 in fig. 7, and exhibits the mandible and dentition tolerably well 

 preserved, except at the anterior and posterior extremities. The 

 two rami occupy only one half the entire length of the jaw, the 

 anterior half being formed by the enormously elongated presym- 

 physial bone. Each ramus is narrow and deep, gradually tapering 

 in front, and the upper border rises behind, immediately beyond 

 the posterior termination of the tooth-bearing portion. The two 

 rami meet in front at a very acute angle, but imperfections in the 

 fossil do not permit of a determination of the precise characters of 

 the union : the symphysis is elongate, gradually diminishing to a 

 thin edge below, and the enormous presymphysial bone is articu- 

 lated to the sloping triangular surface thus formed (fig. 8). The 

 last-mentioned bone is a median unpaired element, very gradually 

 tapering to a point anteriorly: it is hollow, compressed below (fig. 9 a), 

 keeled inferiorly in the anterior portion, and marked by a shallow 

 longitudinal channel above. 



As shown both by this specimen and by another less perfect fossil 

 (fig. 9), originally figured by Dixon, the presymphysial bone is pro- 

 vided with a powerful prehensile dentition. There is a median row 

 of large conical teeth, widely and irregularly spaced, but smaller 

 and more closely approximated in front than behind. These teeth 

 are nearly thirty in number, and each has the form of a hollow 

 cone, enamelled to within a very short distance of the base 

 (fig. 10). The greater length of the enamelled portion is delicately 

 striated, but near the tip the tooth suddenly tapers more rapidly 

 and is smooth. A great number of similar teeth of small size — the 

 largest being little more than one sixth the height of the largest of 

 the median series — are placed irregularly upon the lateral margins 

 of the bone ; they are closely clustered together in more than one 

 row, and perhaps somew^hat more pointed than the median teeth. 

 Like the latter, they are not placed in sockets, but merely anchylosed 

 to the bone. 



The large median teeth end abruptly at the posterior extremity 

 of the presymphysial element ; but the small lateral teeth are con- 

 tinued backwards upon the rami of the jaw, increasing in size and 

 at the same time becoming relatively shorter and adapted for crush- 

 ing. Anteriorly, for a very short space, the dentigerous margin is 

 evidently formed by the true dentary (c?) ; but this bone is soon ex- 

 cluded from the upper border, and the large splenial element (spl) 

 takes its place. The suture between the two bones is well shown 

 in the side view (fig. 8 s). Minute teeth extend far downwards 

 upon the inner side of the splenial, and as the dentition becomes still 

 more adapted for crushing behind, the upper margin of the element 

 is considerably widened, until the anterior width of 2-5 millim. has 



