GO MESSRS. HANCOCK AND ATTKEY ON 



" characterized by a raised longitudinal rib," differ from those 

 of Ctenodus, which have no such process or "rib." 



All the tooth-plates described in the sequel of this communi- 

 cation have such a general resemblance to each other that there 

 can be no doubt of the close relationship of the fishes to which 

 they belong. And, moreover, the bones to which many of them 

 are found attached closely resemble each other. The palatal 

 tooth (or that which has been so designated) is seated on a 

 broadish flattened bone, which, with one exception, is never 

 more than twice the length of the tooth, and is usually consider- 

 ably shorter ; and it is always much expanded at the posterior 

 extremity. This is probably a maxillary bone ; and the two 

 branches, when united, have much the appearance of an upper 

 jaw. The propriety, therefore, of calling these palatal plates or 

 teeth may be questioned. The homologies, however, of these 

 parts must be left for the present untouched.* The mandibular 

 tooth is always much narrower than the palatal ; and the 

 branch of the mandibulum on which it is placed is not so wide 

 as the bone supporting the palatal tooth, neither is it so much 

 expanded at the posterior end ; it is, however, strong and rather 

 massive. 



There are two species that have the surface of the teeth with 

 smooth ridges not tuberculated or denticulated, as they are in 

 all the other kinds. These two have consequently some resem- 

 blance to the palatal plates of Ceratodus, apparently a closely al- 

 lied form. When further research shall have thrown more light 

 on these obscure species, it will then be time enough to consider 

 the desirableness of dividing this apparently natural group into 

 separate genera. 



We shall now conclude this brief communication with concise 

 descriptions of the various dental plates that have come under 

 our observation, retaining the denomination of palatal plates or 

 teeth for the upper pair, though the bones to which they are 

 attached have all the appearance of maxillaries, and so they will 

 be named in the following descriptions. 



* This matter is undoubtedly discussed in Professor Pander's "Monogrniih on the Cteno- 

 (lodipterini ; " but we have not seen that important work. 



