1866.] YOUNG PLATTSOMFS. 311 



Etjrysomtjs = Plattsomus. Ag. jpartim. 



This name is proposed provisionally for PI. macrurus, since the 

 character of its dentition removes it from Platysomus and Amphi- 

 centrum, with which it shares the common want of a ventral fin. 

 The scale-character allies it somewhat to the latter genus. Not 

 having seen, however, the characteristic dentition, I refrain from 

 offering any generic definition*. 



The relations of these three to each other and to other genera are 

 somewhat complicated. The structure of the head is, in all three, 

 strikingly like that of many osseous fishes, and illustrates the inex- 

 actness of the group of Ganoids, especially when internal anatomy is 

 not available for purposes of classification. The palato-pterygoid 

 arch, whose details seem so well brought out in the head of Platy- 

 somus, is not a little remarkable for the share it takes in the outer 

 configuration of the head. In fact, if the large cheek-plates of Trigla 

 are removed, the head of Platysomus is presented, with the single 

 exception of the maxilla, which is a narrow bar in place of the ex- 

 panded triangular plate of the fossil. If this plate is regarded as a 

 facial plate, and the maxilla restricted to the slender spatulate piece 

 marked off by a line near its lower margin, the peculiarity is not 

 less, since a suborbital attached to the maxilla and premaxilla (for 

 these two are closely united) is equally remarkable. The articulation 

 of the maxilla with the pterygoid plates is not quite certain — they 

 overlap and are closely related at least ; but even their articulation 

 could not be regarded as wholly exceptional, since it only differs 

 from the similar arrangement in Crocodilia by the absence of an os 

 transversum. To Trigla a further physiological resemblance may be 

 noted. The great size of the buccal cavity is obtained in both by 

 the same means, namely, the lateral extension of the palatine arcade, 

 and in both doubtless has reference to the predatory habits of the 

 animal. The head of Platysomus is narrow, but every available 

 space is secured for the mouth by the tenuity of the pterygoid appa- 

 ratus ; in fact, only the thickness of these bones is to be subtracted 

 from the transverse measurement to give the entire space available 

 for food. In Amphicentrum, on the other hand, the hard food, to 

 whose attrition the tubercle -teeth are adapted, required less space; or, 

 more correctly, required closer approximation of the crushing sur- 

 faces. Hence the broader jaws, low -set palate and contracted cavity, 

 though the gape is wider than in Platysomus. In this genus, there- 

 fore, the palato-pterygoid arcade recedes from the surface of the 

 head, and is concealed by a covering of skin, or possibly by the de- 

 velopment of membrane-bones. In no point is the difference of the 

 two genera more striking than in the maxilla ; and the light which 

 that bone in AmpJiicentrum throws upon the anatomy of Pycnodus, 

 gives it peculiar interest. In Platysomus it is as in Paloeoniscus and 

 other Lepidosteids — a single thin bone with small laniary teeth, 



* The scale-ornament of PI. striatus manifests, in many specimens, a tendency 

 to the granular form. The figure substituted for that of this species in the 

 ' Poiss. Fossiles,' pi. 17, is accompanied by no description of the conical teeth 

 figured. In the meantime, therefore, it must remain among the Platysomi. 



