58 



although there is a faint angularitj^ which extends from the beaks to the 

 posterior end of the hinge margin. The posterior end is brolven but it 

 seems to have descended in a gently convex, oblique curve to the ventral 

 margin, which is very broadly rounded. In front of the beaks, but below 

 them, there is a concave, but not very deeply excavated or definitely 

 margined, lunular area ; the anterior extremity is uarrowl}' rounded, but 

 slightly angular below. 



Surface strongly and concentrically ribbed ; the ribs rather obtuse and 

 sejjarated by deep, concave grooves. 



Greatest length of the shell, one inch and eight lines ; height, one inch 

 one line and a half; maximum width or thickness, not quite eight lines. 



The only specimen is a somewhat distorted cast, which is imperfect at 

 the posterior end. 



The shell is provisionall}^ ijicluded in Agassiz's genus Pleuromya, as 

 restricted or re-defined by Terquem, on account of its general shape and 

 strong concentric costation; although it may be a Panopoea. Morris and 

 Lj^cett, in their monograph of the Great Oolite MoUusca, and other writers 

 who have acce])ted their conclusions, have reunited Pleuromya with 

 Myacites of Schlotheim, and group the latter genus in the Anatinidas. 

 Stoliczka considers the former part of this hypothesis to be an " inadmis- 

 sible generalization of characters," and believes that Terquem has 

 sufficiently proved the distinctness of Agassiz's genus Pleuromya. Pictet 

 in his " Traitd de Paleontologie," (Vol. III., p. 360) goes still further 

 than Morris and Lj'cett, and unites Myopsls, (Agassiz), Pleuromya, 

 (Agasssiz), Ilomomya, (Agassiz), and some other genera, with Panopcea ; 

 he also places the latter genus (witli Pholadomya) in his fami Ij' Mj^acida^ 



Admitting, for the present, that Pleuromya, niiiy be a good genus, it 

 seems to be capable of division into two well-marked sections. In the first, 

 the beaks are placed very far forwards, and the surface is strongly c(jstate ; 

 in the second, the umbones are situated near the middle, and the valves 

 are only striated concentrically. Pleuromya Carlottensis probably belongs 

 to the first of these divisions, which contains some species which have 

 been referred to Gresslya. 



The whole of the Mesozoic Anatinida3 or Mj-acidie, (for the same genera 

 have been placed in both families b}^ difiereut writers), are very imper- 

 fectly understood, nor is this circumstaice to be wondered at. Although 

 abundant in and eminently characteristic of the rocks of that epoch, the 

 specimens usually metwith are little more thanbadljqn-eserved casts, from 

 which the whole of the thin test has been removed. The microscopical 

 characters of the shell, the nature of the hinge teeth and of the muscular 



