part 4] CAMBRIAN HORIZONS OP COMLEY. 333 



Through the kindness of Dr. Walcott, I have had an oppor- 

 tunity of examining several specimens of the species from Bic, 

 Quebec. The only difference that has been detected is that the 

 concentric ridges in the Canadian specimens are rather smaller and 

 more numerous than those on shells from Comley of the same size. 



Locality and horizons. — Comley, Lower Cambrian; from 

 the Gallavia Sandstone, Olenellus Limestone, Microdiscus-helli- 

 marginatus Limestone, and Protolenus Limestone : horizons Ac p 

 Ac 2 , Ac 3 , & Ac 5 . 



Micromitra (Paterina) minor, sp. nov. (PL XXI, figs. 14-16.) 



(1) Iphidea Bella Billings, Lapworth, 1888. Geol. Mas. p. 485. 

 Iphidea (?) Cobbold, 1911, Rep. Brit, Assoc. (1910, Sheffield), p. 298. 



A form of the genus that is much smaller and more convex than 

 31. (P.) labradorica, is found associated with it in the Olenellus 

 Limestone of Comley. 



Type-specimen [755]. 



Diagnosis. — Ventral valve: outline subquadrate, wider than 

 long, lateral angles slightly rounded ; convexity strong, anterior 

 and lateral slopes gently convex ; apex fairly sharp, a little incurved, 

 and overhanging the posterior border ; shell-substance corneous, 

 exfoliating in layers which mark stages of growth ; surface marked 

 with rounded concentric ridges usually very regularly disposed, but 

 sometimes two or more appear to coalesce so as to form one ridge ; 

 they are continuous round the lateral angles on to the false area ; 

 false area with a wide, open clelthyrium (fig. 14) ; pseudodeltidium 

 either not present at all, or represented only by a minute projection 

 under the apex, which has the appearance of being the posterior 

 margin of the umbonal shield. Internal markings very minute. 

 Three specimens, a little exfoliated at the apex, exhibit a more 

 or less flattened space under the umbo ; on the anterior slopes 

 are traces of radiating corrugations, and also two rather stronger 

 ridges that are slightly sinuous and divergent ; in one of the 

 specimens [750] (tigs. 16 a-16 c) a slightly-elongated central 

 prominence on the cast may be compared with the ' apical slit ' 

 mentioned by Dr. Walcott (1912), pp. 334, 335, 345. 



Dorsal valve, not identified. 



Dimensions. — Four ventral valves have the f olio wing measure- 

 ments, in millimetres. 



[750] [755] [858] [2178] 



Length 1-25 2-0 2-1 1'5 



Width 1-75 2-7 3-25 2-0 



Height 0-5 1-0 1-1 0-6 



Comparisons with other species. — M. (P.) minor suggests 

 M. (P.) bella ' (olim Iphidea oellu Billings) at first sight; but 

 the absence of any well-marked pseudodeltidium at once precludes 

 this reference. 



1 C. D. Walcott, 1912, pi. ii, figs. 1 & 1 a. 



