332 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 31, 



radiatus'^ agree with the Pi/cnodi m the hmitation of the articulated 

 scales to the anterior part of the body. I am inclined to consider 

 this character of generic value ; and I find it associated with other 

 peculiarities in the position and form of the fins and tail, which 

 suggest the propriety of adding Microdon hexagonus and Microdon 

 analis to the genus Gyrodus. Count Miinster has described a Pyc- 

 nodont jaw found by Herr Althaus in the Kupferschiefer at Richels- 

 dorf, which he named Globidodus elegans\. In alluding to this 

 genus Agassiz says that it is probably founded on the dentition of 

 the genus PlatysomusXi a surmise which is proved by Mr. King's 

 specimen of Platysomus macrurus to be perfectly correct. The 

 genus Globulodus must therefore be cancelled. It is probable from 

 the small size of the oral aperture and the character of the dentition, 

 that the Platysomi fed either on marine plants or on small shell-fish 

 or zoophytes, and we trace in their dense tegumentary investment a 

 kind of scale-armour to protect them against the aggressions of the 

 Acrolepides and Pygopteri and other voracious Sauroids with which 

 they co-existed ; but that even this protection did not always avail is 

 substantiated by the fact, that the Globidodus jaw of Count Miin- 

 ster was discovered in a Coprolite ! ! 



3. On Neritoma, a fossil genus of Gasteropodous Molhisks allied 

 to Nerita. By John Morris, Esq., F.G.S. 



Among the fossil shells of the oolite hitherto referred to Nerita, 

 there occurs a small group presenting characters of sufficient im- 

 portance to justify their being separated, not only as a distinct section 

 of the genus, but as forming a different generic type, probably be- 

 longing to the same family, for which I propose the name Neritoma. 



The peculiar character to which I allude is, in the outer lip (which 

 in all the typical Neritce is entire) having two more or less deep 

 sinuses, probably corresponding to a particular organization in the 

 animal inhabitant ; the form of the aperture and the columellar lip 

 are also distinct from those of Nerita, and do not approximate it to 

 any other described genus. 



The above-mentioned characters of this genus, — certainly allied to 

 Nerita, although aberrant from it, — are interesting under two points 

 of view : first, as connecting the true Nerites with Amphibola, Schum. 

 (Atnpidlacera, Quoy), also an aberrant form of Ampullaria and 

 Natica ; and secondly, as adding another instance to certain genera of 

 Mollusca, which with analogous forms present a similar character in 

 having a greater or less sinus in the outer lip. In this latter respect, 

 Neritoma bears the same relation to Nerita, as the other genera first 

 mentioned in the following list do to their respective analogous forms. 



* Poiss. Foss. tab. 69^ figs. 1 & 2. 



t Beitrage, &c. pt. 5. p. 47. pi. 15. fig. 7. 



X Poiss. Foss. vol. ii. pt. 2. p. 203. 



