346 ME. E. S. COBUOLD OX THE [vol. lxxvi, 



margins, and on the greater part of the false area, is covered with 

 rugose concentric ridges, which give place to granulations on the 

 central parts of the shell. 



Of the dorsal valve Dr. Walcott {op. cit. p. 642) says : — 



' On some shells a slight depression or flattening extends (from the apex) to 

 about the middle of the ralve.' 



In the Comley specimens this feature is seen in exfoliated 

 dorsal shells (fig. 22 a). If, however, the successive laminae are 

 removed, the depression becomes increasingly distinct, and, when 

 the surface of the internal cast is reached, there is a distinct central 

 hollow indicating a well-developed ridge or septum on the interior 

 of the shell reaching nearly as far as the anterior border (fig. 22 b). 



The characters of the exterior of the two valves are somewhat 

 dissimilar. The dorsal valve seems to be covered with rugose 

 concentric ridges, like those on the margins and false area of the 

 ventral. 



Dimensions. — Greatest length observed = 9 mm.; the usual 

 length = 5 to 6 mm. 



Locality and horizon. — Comley, Middle Cambrian; from the 

 Billing sella Beds, horizon Be ; from the Paracloxides-davidis 

 Flags, horizon Bb_ ; and from the Orusia Shales, horizon Ca. 



Acbotbeta Kutorga. 



ACEOTEETA SAGITTALIS Salter. 



Obolella sagittalis Salter, 1866, Rep. Brit. Assoc. (1865) p. 285. 



Acrotreta (Linnarssonia) sagittalis Salter, Matley, 1911, Q. J. G. S., vol. lxvii, 



p. 302 & pi. xxvi, figs. 10 a-Wb. 

 Acrotreta sagittalis Salter, Walcott, 1912, U.S. Geol. Surv. Monogr. vol. li, 



p. 701 & pi. lxxi, figs. 2, 2 a-2 h, 3, 3 a-3j. 



A few valves that are referred to this species occur in association 

 with A. sclimalenseei var. at the two sections on the Shoot-Bough 

 Boad, and in the Billingsella Bed of Caradoc Dingle, horizon Be. 



Aceoteeta schmalexseei Walcott, var. matleyi nov. (PI. 

 XXII, figs. 26-30, 32, & (?) 31.) 



Acrotreta aft", socialis Seehach, Matley, 1911, Q. J. G. S. vol. lxvii, p. 303 & 



pi. xxvi, figs. 11 a-13 b. 

 Acrotreta sclimalenseei Walcott, 1912, U.S. Geol. Surv. Monogr. vol. li, p. 709 & 



pi. lxx, figs. 1, 1 ff-1 s. 



The study of about a hundred additional examples of the Comley 

 form of this species, which have been obtained from the original 

 rock-bed since Dr. Matley wrote his notes, shows that the ventral 

 valve is persistently different in form from that depicted in Dr. 

 Walcott's figures ; in the absence of direct comparison with 

 Scandinavian sj^ecimens, it seems therefore advisable to refer to 

 them under a varietal name. 



In the specimens examined by Matley, the anterior margins of 

 the ventral valves were never satisfactorily cleared of matrix ; with 

 the new material available it has been possible to do this in man}^ 

 instances, and to establish the fact that the apices are always 



