138 PHYLLODUS. 



geal bone, like that of the Scarus, or of the maxillae, as in the Diodon, 

 or whether they belong to the hyoid system, or other median bone 

 of the cavity of the mouth ; as, however, it is the structure of the 

 teeth and not of the cranium that it is proposed to describe in this 

 place, the ambiguity which may attach itself to the precise homology 

 of the supporting bone is of less consequence. 



55. Pisodus.{l) — The form of the teeth scattered over the surface of 

 the large flat oval dentigerous bone, figured in PI. 47, fig. 3, suggested 

 the name of the extinct genus of fishes, which the present remarkable 

 fossil unequivocally indicates. It measures four inches and a quarter 

 in length, two inches and a quarter in breadth, and seven lines in 

 depth at the thickest part, which is at the middle of the plate. The 

 teeth are distributed irregularly over the whole of the upper surface, 

 and in pretty close contact, except where they have been displaced 

 by attrition, as from the large flat and polished surface at the anterior 

 part of the bone, or where they have been broken or shed, at the in- 

 terspaces of the unworn teeth. These all present the same form and 

 nearly the same size : they have a hemispherical smooth and polished 

 crown, and are inserted each by a large short hollow conical fang or 

 base into a socket of corresponding form and dimensions. The crowns 

 of some of the teeth project two lines beyond the surface of the bone, 

 others are just protruding above that surface ; indicating that they are 

 shed and renewed in vertical succession : the middle of the edentulous 

 anterior surface is worn smooth and flat, but its circumference is 

 pitted with the remains of the sockets. On the under surface of the 

 dental plate there is a regular elliptical space occupying four-fifths of 

 that surface, and defined by a raised margin ; this indicates the place 

 of attachment of the plate to another bone of the skull, most 

 probably, as in the Glossodus and Sudis to a median bone of the 

 hyoid system. 



The teeth of the Pisodus are adapted for crushing shells, Crusta- 

 cea, or vegetable substances ; their texture is extremely dense, and 

 corresponds with that of the teeth of the Microdon. 



56. Phyllodus.{2) — The dentigerous plates, which have sug- 

 gested to Professor Agassiz this name for the extinct species of 



(1) iriffov, a pea, oSnQ, a tooth. (2) (pvXKov, a leaf, o^«C, a tooth. 



