332 



MR. E. S. COBBOLD OX THE 



[vol. lxxvi, 



Micromitra (Paterixa) labradorica (Billings). (PI. XXI, 

 figs. 10a-13.) 



Oholus labradoricus Billings, 1861, Geol. Surv. Canada, Pal. Poss. vol. i, p. 6. 



fig. 6. 

 Micromitra (P.) labradorica Billings, Walcott, 1912, p. 347 & pi. ii, figs. 2, 



2 a-2f. 

 (?) Kutorgina cingulata Billings, Callaway, 1878, Q. J. G. S. vol. xxxiv, p. 759. 

 (?) Do. do. do. Lapworth, 1888, Geol. Mag. p. 485. 



(?) Do. do. do. Lapworth, 1891, Ibid. p. 532. 



(?) Kutorgina cingulata Billings, Lapworth & Watts, 1894, Proc. Geol. Assoc. 



vol. xiii, p. 310. 

 Kutorgina and Kutorgina cingulata Cobbold, Brit. Assoc. Rep. 1909, p. 182 ; 



i910a, p. 118; 1912, pp. 138, 140, 141. 



It seems probable that the specimens from Comley which have 

 previously been referred to K. cingulata belong in reality to 

 Billings's second species, now included by Dr. Walcott under the 

 genus Micromitra, sub-genus JPaterina. 



The Comley specimens usually have a black outer surface : that is, 

 covered with rounded concentric ridges, spaced, in the anterior part 

 of the valves, at from 4 to 8 to the millimetre. Where exfoliation 

 has taken place the internal laminae of shell are marked by radiating 

 folds, which extend to distances that vary with the degrees of 

 exfoliation. 



Two specimens are known in which the dorsal and ventral valves 

 are united. One of these [B 40306] in the British Museum (figs. 

 10ff-10c) is now much damaged, and it is impossible to ascertain 

 whether the apex is as much curved down at the back as is repre- 

 sented in the side view, the sketches for which were made before 

 the other specimens were studied. It had the two valves slightly 

 displaced one upon the other. In the other specimen, [730] in the 

 collection for the British Association, the shells are somewhat 

 crushed together, and they have suffered much from weathering. 



The dimensions of five dorsal and three ventral valves are as 

 follows, in millimetres : — 





Dorsal Valves. Ventral Valves. 



Nos. on specimens 



[B 40306] 

 4'5 

 6-5 

 0-7 



[730] 



about 9 



about 10 



1-4 



[2188] 

 4-5 

 7-5 

 1-0 



[2179] 

 6-0 

 8-5 

 2-5 



I 

 [1519]![B40306] 



4-2 5-0 



[730] 

 9-5 



11-0 

 1-6 



[729] 

 7 

 10 

 o 



Width 



5-7 

 1-5 



6-5 

 1-3 



Height 





The concentric ridges of the exterior surface can often be traced 

 round the postero-lateral angles of the shell on to the false area. In 

 the dorsal valves the central part of the false area appears to have 

 been widely open, no trace of a pseudochilidium having been 

 detected. In the ventral valves the pseudodeltidium is occasion- 

 ally seen, and takes the form of a very small, horizontal, semicircular 

 shelf (fig. 13). 



