98 Mr. R. Etheridge on Cai-boniferous LamelUhranchiata. 



appear referable to A. ccelatus. Amongst the twenty specimens 

 before me, there are perceptible, although slight, variations in 

 the general form of the shell, number and strength of the ra- 

 diating costse, and relative development of tlie concentric and 

 regularly frilled imbrications. When full-grown the shell 

 must have attained a considerable size, and, with an increase 

 in age, appears to have become longer than high, with a cor- 

 responding widening of the interspaces between the radiating 

 costse and frilled imbrications. In all, the anterior ear of the 

 right valve is elongate and well divided from the body of the 

 shell, although the radiating ridges on it are not constant in 

 number; whilst on the pointed posterior ears of some the 

 frilled imbrications of other parts of the shell become flattened 

 or depressed and not so broken up. In all the right valves 

 the radiating costse are less sharp, and the concentric imbri- 

 cations much flatter and less frill-like than in the left valve, 

 giving to the valve a somewhat reticulate appearance. The ma- 

 jority of our specimens, so far as I can ascertain, exhibit from 

 fifty to fifty-five radiating ribs or costse. The prismatic shell- 

 structure is often visible in a fine state of preservation. The 

 shells I have here referred with doubt to A, ccelatus differ 

 slightly from Prof. McCoy's description of the latter, and at 

 the same time they exhibit characters not shown in the figure 

 of that species. Unfortunately Prof. McCoy's specimen ap- 

 pears to have been defective in the anterior ears ; so that we 

 labour under some difficulty in referring specimens to this 

 species, and a certain amount of latitude must be allowed. 

 With the view of comparing our specimens with examples of 

 P.7 jimhriatus^ Phill., Prof. T. M'K. Hughes was kind enough 

 at my request to forward me specimens from the Cambridge 

 collection, which appear to be those used by Prof. M'Coy for 

 his emended description of Phillips's species. The ornamen- 

 tation, as described by the former, consists of somewhat in- 

 distinct obtuse ridges, crossed at intervals by flattened or de- 

 pressed concentric imbricating growth-lamellae, with still finer 

 lines between them, almost assuming the character of strise. 

 These, with the short obtuse posterior ears and square left 

 anterior ear, extending as far as the edge of the shell, will at 

 once serve as characters whereby A. ccelatus and the present 

 shells on the one hand, and P.'^ Jinibriatus on the other, may 

 be distinguished from one another. 



A similar style of ornamentation of the left valve to that de- 

 scribed above in the shells which I have referred to A. ccelatus 

 is met with in A. subfimhriatus^ D'Arch. and De Vern.* 



* Murchison's Geol. Russia, vol. ii. Pal^ontologie, p. 327, t. 21. f. 6. 



