HIPPONYX* 



iLre, therefore, obliged to consider it as an equilateral in? 

 equivalve, rather irregular bivalve; there is a muscular 

 impression in each valve of the general form of a horse- 

 shoe ; the fibres of which this is composed are placed in a 

 different direction from those composing the remainder of 

 the shell, and appear to be more easily decomposed, for 

 which reason there are frequently found fossil specimens 

 with cavities in the place originally occupied bv the mus- 

 cular impression, particularly in the lower valve^ which, in 

 its general form, is orbicular, compressed, sometimes very 

 much thickened, attached by its outer part to submarine 

 bodies, its margin is for the most part elevated, more so in 

 front than behind; its muscular impression can scarcely be 

 called single, the two semilunar portions which compose 

 it are placed close together; they are nearly confluent and 

 narrow at the posterior part; more distant in front, where 

 they are broader and rounder : it, however, has the gene7 

 ral form of 9^ horse-shoe. The upper valve is patelliform, 

 subconical, sometimes very much compressed; its vertex or 

 umbo is inclined backwards, and towards the posterior 

 margin ; like the attached valve it has an internal muscular 

 impression in the shape of a horse-shoe, placed near the 

 posterior margin, the two lobes of which are much more 

 distant and obliquely truncated in front, but entirely con- 

 fluent at the posterior part : there is a remarkable coinci- 

 dence in the muscular impressions of this genus, with those 

 of Terrebratula^ which, in the lower valve, has two, placed 

 near to each other, and in the upper valve a central one 

 composed of two confluent portions. In Hipponyx there 

 is no cardinal ligament, nor are there any teeth. 



It was from the circumstance of observing a cast of the 

 interior of the upper patelliform valve attached to a lower 

 valve that De France began to suspect the true nature of 

 this shell; his suspicions were afterwards confirmed by his 

 finding the two valves together on the outside of a specimen 

 of Cerithium Cornucopia; and also by the discovery of an 

 attached valve of Ilippmijx mitratus. About the same time 

 we were almost persuaded of the same fact by an examina- 

 tion of the upper valve, and a comparison we instituted^ 

 between it and some specimens of Orbicula and Crania" 

 which had just then fallen into our hands; we had not, 

 however, the means of proving the truth of our surmise ; 

 l^ut we have since met with several specimens of a small 

 fj[>ssil species, the Ilipponijx Icevisj in the inside of other 



