165 



CucuLL^A (Idonearca) TRUNCATA, CtABB. 



Plate 19, figures 2 and 2a. 



Cucullxa truncata, Gabb. —Pal. Cal., Vol. I., p. 196, pi. 25, flg. 182. 



?z= C. (IdoTiearca) Nebrascensis, Owen. — See Meek's Rep. Cret. and Tert. Foss. U. 



Miss. Co., p. 88, pi. 29, figs. 5, a, b. 

 ?=aC. (^Idonearca) Shumardi., M. & H. — See the same work, p. 86, pi. 28, figs. I5a-g, 



and pi. 29, fig. 4. 

 Possibly a var. of C. glabra, Parkinson. — For the synonymy of this species see Pictet & 



Campiche's Pal. Suisse, Foss. Ste. Croix, 



Vol. III., p. 456. 



Middle Shales, Division D, of the west side of Hornby Island ; Lower 

 Shales, Division B, of Gabriola Island ; Productive Coal Measures, 

 Division A, at Nanairao River, two miles and a-half up, and North West 

 Bay, Y. I. ; also at the Sucia Islands ; J. Richardson, ISIl-TS. 



This species is subject to considerable variation, both in shape and 

 sculpture. In some specimens the margin of the posterior end is 

 truncated almost vertically and the jjosterior side is shorter tiian the 

 anterior. Others have almost exactly the same outline as the " mature 

 specimen" of CucuUcca Nebrasce7isis figured b}' Meek, and in these the 

 posterior side is longer than the anterior, the former being produced and 

 very obliquely truncated at its termination. Young shells, about an 

 inch in length, are marked by crowded radiating ribs, which are nodulous 

 in consequence of their being crossed by equally numerous, raised, 

 concentric stria?. In adult individuals the radiating ribs are never 

 nodulous, and sometimes become nearly obsolete. The left valve is 

 invariably larger than the right. The height, as measured from the top 

 of the beaks to the base, is usually a little less than the length, and 

 slightly greater than the thickness through the closed valves. The 

 test of aged shells is upwards of four lines in thickness. On the inside 

 of the test there is a prominent, sharp ridge, which bounds the inner 

 side of the posterior muscular impression. The hinge plate is broad at 

 the ends and narrow in the middle ; it bears three or four elongated 

 teeth at each end, and a few rather small, transverse teeth or denticles 

 in the centre. The lateral teeth, which are striated or grooved across, 

 are parallel with the cardinal margin, and their inner ends are appa- 

 rently not bent downwards. The central teeth or denticles are each as 

 wide as that part of the hinge plate on which it is 2:)laced, and are not 

 minute, granular, or arranged in two or three rows which ascend 

 obliquely upwards and outwards, as in Nemodon. Some of tbese central 

 denticles are little more than raised, rounded tubercles. 



