PRODUOTUS. 153 



before him, but it would scarcely be equally applicable to all the shells which compose the 

 species. Thus, the median depression is extremely shallow, and even absent ill certain 

 examples ; the characteristic rugged, semicircular ridge, with its strong, elongated spines, 

 present on typical specimens, is at times very little developed, and even absolutely wanting 

 in certain examples, while the number of ribs will vary from eighteen to fifty, and are 

 more often rounded than flattened, the spines being likewise more numerous on some 

 examples than on others. When publishing my monograph of Scottish Carboniferous 

 Brachiopoda I erroneously placed Sowerby's P. .sulcata* among the synonyms of P. semi- 

 rcticu/atus, but a more attentive examination of the subject has convinced me that Phillips 

 was correct when, in 183G, he placed P. sulcatum among the synonyms of the species under 

 description. 1 M'Coy's P. cost ell at us is evidently a synonym of P. costatus, but it is more 

 difficult to determine whether P. muricatus and another closely allied form from Corrieburn, 

 in Stirlingshire, should be considered as varieties of Sowerby's P. costatus. Having 

 obtained the loan of the original example of P. muricatus preserved in the museum at 

 York (fig. 11 of my plate), I found that it was imperfect, and much more circular than the 

 typical and usual shapes of P. costatus ; but it presented, however, the same median depres- 

 sion, and although the tubular spines projecting from the convex surface of the ribs were 

 more numerous, still intermediate forms seemed to unite Phillips's species to P. costatus 

 proper. I am therefore inclined to believe that P. muricatus is only a variety of the shell 

 under description, or a race peculiar to certain localities ; but as I may be mistaken, and 

 in order to allow the subject to remain an open question, I will describe Phillips's shell 

 under a separate or varietal denomination. 



Far. i P. muricatus, Phillips. PI. XXXII, figs. 10—14. 



Producta muricata, Phillips. Geol. of York., pi. viii, fig. 3. 



Shell circular, about as long as wide ; hinge-line somewhat shorter than the greatest 

 width of the shell ; ventral valve regularly arched, so as to present in profile nearly a half 

 circle, uniformly convex, or slightly longitudinally depressed along the middle ; beak large, 

 wide, rounded and incurved, but not overlaying the hinge-line except at its attenuated 

 extremity ; ears very small ; surface covered with numerous rounded ribs, of unequal width, 

 augmenting in number here and there by bifurcation or intercalation, while two or three 

 will sometimes unite into one ; long, slender spines project at intervals from the ribs, and 

 form also a row close to the cardinal edge. Dorsal valve regularly concave and similarly 

 sculptured. Proportions variable. Two specimens have measured — 

 Length 11, width 10, greatest depth 5 lines. 



1) J- "2 >* " » " 



1 As already mentioned under P. semireticulatus, Mr. Salter is inclined to consider the last-named 

 shell and P. costatus as varieties of a single species. 



20 



