﻿Sir Philip Grey-Egerton — On some Neio Pycnodonts. 51 



small teeth are intercalated between the rows, in others the larger 

 teeth are replaced by two smaller ones, as in the specimen described 

 in this article ; but in every case these teeth are smaller than those 

 of the principal row. In the genus Gyrodus the dental formula 

 does not appear to be subject to these irregularities, there being con- 

 stantly four rows in each mandible, and five in the vomer. In this 

 genus the outer row comes next in size to the principal row, and all 

 the teeth have a central eminence surrounded by a fossa. In Microdon 

 also the teeth of the outer row of the four are next in size to the 

 principal ones. 



PalcBobalistum of de Blainville has three rows, the component 

 teeth being arranged obliquely. In Mesodon of Wagner the teeth 

 are oval, concave, and notched on the periphery. HeckeP has added 

 two genera to the family, — Stemmatodus having three rows of teeth 

 on either side with notched borders and granulate crowns ; and 

 Coelodus described as having three rows on each mandible, elliptical, 

 with a slight depression on the surface of each tooth. He has, 

 moreover, upon the evidence of anatomical structure combined with 

 the dental characters, remodelled the arrangement of the species of 

 this family. 



Microdon hexagonus and riigidosus of Agassiz are referred to 

 Gyrodus, as is also Microdon truncatus of Wagner. Pycnodiis Itieri, 

 Sauvanasii, Bernardi, Egertoni, and Wagneri of Thiolliere ; Pycnodus 

 umhonatus and Sugii of Agassiz ; and Pycnodus formosus and Beussii 

 of Wagner, are removed to the genus Microdon. Gyrodus macropterus 

 of Agassiz, and Pycnodus liassicus mihi, are joined to Mesodon. 

 Pycnodus rJiombus of Agassiz goes to Stemmatodus, and Pycnodus 

 orhicidatus of Agassiz to its pristine genus Palceobalistum of de Blain- 

 ville. The new genus Coelodus, in addition to the many fine species 

 of entire fish found in Austria, is made to include Pycnodus rhombus, 

 and Glossodus angustatus of Costa and Pycnodus MantelU of Agassiz. 



On comparing the Folkestone fossil with these several forms, 

 there is no doubt but that it mostly resembles the genus Ccelodus of 

 Heckel. A detached tooth from the outer row might indeed be 

 mistaken for a Gyrodus tooth if the crown were at all ground down, 

 but in a young tooth with the surface intact the difference is very 

 discernible. In specimens having the ranks of teeth preserved 

 there can be no mistake, as in Gyrodus the teeth of the outer row 

 are next in size to the principal teeth, whereas in Coelodus they 

 diminish jDrogressively from the inner to the outer series. The 

 number of the dental ranks in the hinder part of the jaw agrees 

 with Heckel's formula, and, as I have stated before, I do not con- 

 sider the irregularity of the subsidiary ranks in the anterior portion 

 as indicative of generic variation. Assuming then that this mandible 

 belongs to Coelodus, it is manifest that it cannot be identified with 

 any of the species of the genus yet described. It is of larger size 

 than the Coelodus Snturnus figured by Heckel, and differs from that and 

 all the other species in having the teeth of the second row more 

 numerous and more elliptical. I propose to call it Coelodus ellipticus, 

 ^ Beitrage zur Kenntniss der fossilen Fische Oesterreichs, pt. 2. 



