72 BRITISH CARBONIFEROUS BRACHIOPODA. 



that the study of several specimens of this interesting species has not enabled me to arrive 

 at so decided an opinion, and I am therefore still uncertain as to the propriety of classing 

 the species in question with Pentamerus. 



Professor M'Coy's illustrations of the interior (figs. 13, 14 of my plate) are, as he has 

 himself admitted, evidently imperfect, for the dental plates are not represented extending 

 so far as the inner upper extremity of the large mesial septum, which my preparations 

 (figs. 11, 12) so completely exhibit. The appearance also of the internal cast of the dorsal 

 valve of a specimen preserved in the Museum of Practical Geology (fig. G), would lead me 

 to infer that in the smaller valve there existed but a single median plate,^ instead of two 

 sub-parallel septa, as described and imperfectly represented in the work above quoted (fig. 

 14), and we have no certain evidence that the internal details of this valve were exactly 

 similar to those of Fentamerus ; on the contrary, there exists between the shell under 

 description and C. septosa so much resemblance, both in external appearances as well as in 

 the interior details of the ventral valve, that I have deemed it preferable (or at least so 

 provisionally) to locate both Phillips's and M'Coy's species into my sub-genus Cyrtma, and 

 from which the last-named author's shell may be hereafter removed should the discovery 

 of the internal details of the dorsal valve determine the necessity. 



It is possible that these /iSJt?eV«/(?r«-shaped shells were not provided with spiral 

 appendages; and that they formed a kind of passage between Spirifer and Pentamerus ; 

 but this must for the present remain an unsettled question. 



A large area, similar to that seen in C. carbonarhis, is not a character of Pentamerus^ 

 but it is necessary to remember that rudimentary areas occur in both Pentamerus lens and 

 P. liratus, and nothing can be more variable than the extent and dimensions of the 

 internal plates in different species of the genus. It is, therefore, to be hoped that ere long 

 the discovery of some suitable specimens of C. carhonarius will enable us to determine the 

 characters of the smaller valve, which are so important in the determination, not only of 

 the genus, but also of its position in the classification of the group. 



The figures I have selected for illustration will convey a good idea of the extreme 

 variability in shapes presented by this species. In some examples the beak of the larger 

 valve is so incurved as to come into contact with that of the smaller valve (fig. 5), while it 

 assumes every degree of incurvature from this extreme condition to that in which the area 

 is almost flat (figs. 9, 10). In relative width, breadth, as well as in degree of convexity, 

 great differences are perceptible, as may be inferred from the measurements taken from 

 four examples above noted. The ribs also vary considerably both in width and in the 

 number of bifurcations, trichotomisings, or intercalations they may assume, from twenty to 

 thirty being counted round the margin of each valve in different specimens ; they are also 

 at times much distorted, and, as mentioned by Professor M'Coy, " their surface is rather 

 rugged and very coarsely granulo-punctate or minutely pustular under the lens;" but this 



' A single median slit is observable in tlic cast (fig. 6). Had two sub-parallel srpta existed, two slits 

 would have been visible on the cast. 



