SPIRIFERA. 61 



possible doubt as to the close affinity or specific identity between Sp. oblatus, Sp. obtusus, 

 and Martin's shell ; those of Phillips, therefore, will alone require some further 

 consideration. 



The original specimen on which Sp. symmetrica was founded forms part of the 

 Gilbertsonian collection in the British Museum, and has appeared to Professor De Koninck, 

 as well as to myself, to be a variation of S. glabra, in which the mesial fold is but feebly 

 elevated above the regular convexity of the valve, with also a slight longitudinal depres- 

 sion or groove along its middle (PI. XI, fig. 6). Nothing seems to be more variable than 

 the development of the mesial fold, for it is entirely absent in some young individuals, 

 while in others of a similar age it becomes sharply defined. 



Professor M'Coy has strongly urged the maintaining of Sp. symmetrica as a distinct 

 species, probably from not having had sufficient opportunity of studying the original 

 type, for otherwise he would have seen that it has not those distinctive features he so 

 emphatically announces. "This beautiful and very distinct species varies very little in 

 its characters ; .... it is remarkable for the nearly regular rhomboic outline of the 

 receiving valve ; for the broad, often minutely notched, sinus in the front margin, producing 

 scarcely any distinct mesial ridge ; and for the strong, filiform subregular, distant radiating 

 lines from the beak to the margin of the interior, often appearing on the external surface. 

 Two specimens differ remarkably (one from Lowick and one from Derbyshire), by the hinge 

 line being only tVcj as compared to the width ; and one of them, by the length nearly equal- 

 ling the width, and the shortness of the beak of the receiving valve, so strongly approxi- 

 mates to S. decora, that I suspect additional experience may unite these species. Both 

 differ from all the varieties of Spirifera {Martinia) glabra by the strong threadlike, sub- 

 regular, internal ridging from the beak to the margin." To this I would observe, that the 

 length of hinge line and area in many indubitable examples of S. glabra is not more than 

 a third of the breadth of the shell, and, in such cases, entirely agreeing with what we find 

 in typical shapes of Sp. decora. The rhomboic outhne, also, is not constant in the last- 

 named shell, as a series of specimens from the Island of Man has completely confirmed ; 

 and it is rare to meet with specimens so strongly marked as the one represented (PI. XII, 

 fig. 12), for every passage or intermediate link will be found connecting it with the more 

 common shapes o{ glabra. The double longitudinal groove observable along the middle 

 of the sinus and in the mesial fold is likewise to be seen in many specimens of 

 Martin's shell (PL XI, fig. 1), as well as the fihform, subregular, distant radiating lines 

 mentioned by Professor M'Coy. I therefore agree with the last-named author, while 

 considering Sp. decora intimately connected with Sp. symmetrica, but must also go a 

 step further, by uniting the last-named shell to Martin's species. 



It is worthy of remark, that although the surface of Sp. glabra is in general entirely 

 smooth, in some exceptional cases there is a tendency to the formation of rounded ribs 

 on the lateral portions of the valves (PI. XII, fig. 3), and to this variation must be referred 

 Professor Phillips's Sp. linguifera, of which fig. 4 is a representation, drawn from the 



