FOSSIL FISHES. 29 



is a single or double depression, also like that in Heterostius. This is the 

 bone called by Pander os occijpitale medimn. 



The lateral angles of the head are formed by what Pander calls ossa 

 artiGularia capitis^ shown in Figs. 3a, 3b, and 4, of Plate 34, Vol. I. 

 The posterior angle of this bone is excavated to form a deep, somewhat 

 conical socket, into which fits a strong condyle projecting from the mid- 

 dle of the Snpra-scapnla (Post Temporal), making one of the most com- 

 plete joints known in the animal kingdom. It is strengthened by a 

 gnard, or rest, which projects like a flattened tlnimb, from the margin of 

 the socket, backward, beneath the Supra-scapnla. In all the specimens 

 yet fonnd the " os articulare capitis " is so firmly united to the other 

 bones of the cranium that its outlines cannot be accnratel}^ determined. 

 I have supposed that it represented chiefly the Epiotic, bnt it may also 

 represent this bone inseparably blended with the squamosal and parietal 

 bones. Future discoveries will probably afford the means for deciding 

 this question. 



Dentition. — The " premaxillaries " in D. Terelli are triangular in out- 

 line, but are narrower than those of D. Ilertzeri. They terminate below 

 in a sharp but strong point. The upper margin, instead of being flattened 

 and laminar, as in D. Ilertzeri., is thickened, and the posterior lateral 

 angle sometimes becomes a massive knob. The central position of the 

 upper margin is excavated, forming a kind of socket. The exterior sur- 

 face of the " premaxillaries " is smooth, and without the line of tubercles 

 which ornament those of D. Ilertseri. The inner side is concave and 

 frequently much worn and excavated by the prominent extremity of the 

 mandible, over wdiicli it shuts. Front and lateral views of a complete 

 " premaxillary " of medium size, are shown in Figs. 1 and 2, of Chart V. 



The " maxillaries " are oblong with the angles somewhat rounded. 

 The upper margin recedes, and near the anterior extremity bears a flat- 

 tened process an inch or more in length. The external surface is slightly 

 arched in both directions; it is nearly smooth, but along the lower mar- 

 gin, shows a band of low, flattened ridges (enamel folds). The interior 

 face is concave, and on the lower margin, which is always sharp, it is 

 more or less worn by contact with the knife-edge of the mandible upon 

 which it plays. Figures representing the internal and external aspects 

 of a " maxillary " of D. Terrelli are given in Chart V. (Figs. 8 and 4). 



The " maxillaries " of D. Ilertzeri are less quadrangular in form than 

 in D. Terrelli, the lower margin being considerably the longer. They 

 are also thinner and flatter, and are set with sharp, conical teeth. 



The mandibles of the species under consideration have a maximum 

 length of about two feet, the largest complete one in my posses- 

 sion being twenty-two inches in length. Two others which I have 



