Professor Rupert Jones — Isochilince from N. America. 301 



I. Isochilina gregaria (Whitfield), var. Ulrichiana, noY. (Figs, la-h.) 



No. 1 is a specimen of fine-grained, compact, grey-blue limestone, 

 retaining part of the smooth surface of a suboval pebble, perhaps 

 originally about four inches long. The broken surface (of the split ? 



x2 



Fig. 1. — Isochilina gregaria CWhitfield), var. Ulrichiaiia, nov. («) Eight-hand 



valve, (b) Dorsal edge. Magnified 3 diani. 

 Figs. 2a, b. — Another specimen of the same. Magnified 3 diam. 

 Fig. 3. — Isochilina sp., ind. Crushed. Magnified 2 diam. 



Figs. 1-3 from a drifted (Trenton ?) limestone at Hamilton, Ontario. 



pebble) shows one good example and two smaller indications of 

 a smooth, black, and shiny species of Isochilina, which I regard as 

 a modification of I. gregaria (Whitfield), Bull. Amer. Mus. Nat, Hist., 

 vol. ii, p. 58, pi. xiii, fig. 3. 



It is an imbedded bivalved carapace, showing its dextral valve, 

 ovate-oblong, narrower and more convex in front than behind. '^ 

 Marginal rim strong on the posterior, ventral, and anterior borders. 

 Surface black, smooth, shiny, and very delicately punctate. It has 

 a broad mid-dorsal depression (nuchal notch), bordered by a low 

 semicircular ridge, composed of four tubercles of irregular size ; the 

 two largest touch the dorsal edge, and are united below by one 

 round and two oblong smaller tubercles. 



This nuchal sulcus approximates in the pattern of its border to 

 that in Isochilina gregaria (Whitfield). 



The length of the valve is 8 mm., height 6 mm. It is in the 

 specimen No. 1, ' Bala Drift,' Hamilton, Ontario. 



la. Isochilina gregaria (Whitfield), var. Ulrichiana, nov. 

 (Figs. 2a-b.) 



No. 2 specimen is a piece of limestone similar to that of No. 1, but 

 darker. One face appears to have been artificially flattened and 



1 This anterior convexity is unusual. 



