350 ME. E. S. COBBOLD OX THE [vol. lxxvi, 



Shell-substance in both, valves corneous, thick, and formed of 

 many laminae ; exterior marked by very closely-set concentric striae ; 

 inner layers occasionally showing faint traces of radiating striae. 



Dimensions. — Lengths = frorn 1*9 to 3*25 mm. ; proportion of 

 length to width=about 1 to l'l ; of length to height, in ventral 

 valves=about 1 to '33, in dorsal valves = l to "2. 



Observations. — The internal characters seen in the casts vary 

 so much in size, in proportion to the diameters, as to suggest that 

 more than one species may be present. These divergences are, 

 however, due to the amount of sbell remaining on the cast, and 

 disappear on its removal. 



In fig. 2, PI. XXIII, representing an internal cast of the ventral 

 valve from which all the shell-substance has come away, the bell- 

 shaped cavity extends to about a third of the length of the shell ; 

 fig. 3 represents a cast with some of the shell left on it, and here 

 the cavity extends to about a fifth of the shell-length ; in fig. 4, 

 from a cast to which much shell adheres, the extension is but to a 

 twelfth, and in this case the visceral cavity has the circular form 

 seen in the young shell. Figs. 1 a to 1 e are five views of the apex 

 of one ventral valve, from which four successive layers of shell were 

 removed. In fig. 1 a the exterior of the shell is seen ; in 1 b, after 

 the first exfoliation, the visceral cavity is represented by the circular 

 prominence, while in the succeeding figures the cavity assumes its 

 more normal shape. The various forms are evidently the internal 

 casts at successive stages of growth. 



Variation. — Occasionally a full-sized shell is found, in which 

 the cavity retains somewhat of the circular shape seen in the voung 

 shell (fig" 7, PL XXIII). 



The relations of Acrotliyra comleyensis to other species are 

 discussed with those of A. cf. sera (below). It may. however, 

 be remarked here that indications of the two little pits near the 

 foramen may be seen in the figures of the following species in 

 Dr. C. D. Walcotfs monograph : — 



Acrotreta definita Walcott, pi. lxiv, fig-;. 2 c , 2 c' . 

 Acrotreta kutorgai Walcott, pi. lxv, tigs. 36, 36', 3 p, 3 c' . 

 Acrotreta argenta Walcott, pi. lxvii, figs, ba.ob, ob'. 



They are also seen in A. cf. sera ; see PI. XXIII, fig. 10, of this 

 paper. 



Locality and horizon. — Comley, Middle Cambrian. A. com- 

 leyensis is found associated with Dorypyge JaJcei Cobbold, at two 

 local horizons, Ba T andBa 3 ; in the former, fragments of Paradoxides 

 have also been found. 



Aceothtra cf. sera. Matthew. (PL XXIII, figs. 9 a-18.) 



Acrotliyra signata sera Matthew, 1902. Bull. Nat. Hist. Soc. New Brunswick, 

 vol. iv, pt. 5, No. 20, pp. 383, 384 & pi. xiii, figs. 3 a-3 e. 



Acrotliyra sera Matthew, Walcott, 1912, U.S. Geol. Surv. Monogr. vol. li, 

 p. 718 & pi. lxxx, figs. 4, 4«-4 c, 5, 5«-5 d, 6, Qa-6 i, 7, 7 «-7 h, 8 (?). Beasons 

 are given for grouping several of Dr. Matthew's species or varieties under one 

 specific name. 



A form which is very close to, if not identical with, the Cape 

 Breton species, occurs in considerable quantity in the granular 



