part 4] THE CAMBRIAN HORIZONS OP COMLEY. 353 



Many of Dr. Walcott's figures of Acroihyra sera (op. cit. pi. lxxx) 

 may be matched from the Comley specimens ; hut the vertical groove 

 on the false area in his figures 4 a" & 5" is much stronger than 

 any noted from Comley. 



The description given by Dr. Matthew under A. signata sera 

 (op>. cit. p. 383) applies fairly accurately to the Comley shells. Of 

 the interior he says that it has 



' Near the hinge a thick rectangular callus hollowed at the middle, with a 

 depression that deepens towards the hinge ; from this it is divided by a low 

 transverse ridge, behind which is a pit leading to the foramen.' 



He makes no mention of the connexion between the depression 

 and the foraminal pit, such as is seen in A. comleyensis and in 

 A. cf. sera. In his description of A. signata (op. cit. p. 381), 

 however, he says : — 



' There is a strong narrow callus, one-third the length of the valve, bounded 

 by a raised ridge at the sides and in front ; at the front of this callus is an 

 oval pit from which a groove runs backward nearly to the apex of the shell, 

 where it is supposed to connect with the foramen.' 



In A. proavia, the type of the genus, no mention is made of a 

 connecting groove ; but the hollow is represented in Matthew's 

 figures (op. cit. text-fig. 2, p. 386, & pi. ii, figs. 4>b-5b) as 

 continuous nearly as far as the apex. Apparently in this species 

 the hollow, the groove, and the foraminal pit are merged into one. 



These differences appear to be connected with the elongation of 

 the valve backwards, though which form is the more primitive 

 cannot be determined upon the present evidence. 



Text-fig. 4 (p. 352) is introduced so as to show graphically the 

 modifications here mentioned. 



BILLINGSELLiaffi Schuchert 

 Billingsella Hall & Clarke. 

 Billingsella cobboldi Matley. 



Billingsella cobboldi Matley, 1911, Q.J. G.S. vol. lxvii, p. 305 & pi. xxvi, 

 fig's. 20-23. 



Locality and horizon. — Comley, Middle Cambrian; from 

 the Paradoxides-davidis Flags, horizon Bb_. 



Billingsella lindstrcemi. var. salopiensis Matlev. (PL 

 XXIII, figs. 24 a-24c.) 



Billingsella lindstrcemi Limiarsson, var. salopiensis Matley, 1911, Q. J. G. S. 

 vol. lxvii, p. 303 & pi. xxvi, figs. 14a-17 c. 



Since this form was described by Dr. Matley, a large number of 

 additional specimens have been obtained from the Billingsella Bed 

 of Caradoc Dingle, many being more complete than those figured 

 with his notes. From these it appears that the apex of the ventral 

 valve is recurved over the posterior border to a greater extent 

 than is shown in his figure, hiding the convex deltidium, which 

 has not been observed. 



