ART. 4 CAMBRIAN CONCHOSTRACA — ULRICH AND BASSLER 71 



Occurrence. — Lower Cambrian (Upper Etcheminian-div. E3e, E3/) 

 Dugald Brook, Cape Breton. Occurs also in the Hanfordian C163, 

 Hanford Brook, New Briins-wick. 



Plesiotypes.—Csit. Nos. 56474-56476, U.S.N.M. 



INDIANA SECUNDA (Matthew) 



Plate 9, Figures 5-8 



Aparchites secunda Matthew, Trans. New York Acad. Sci., vol. 14, 1895, p. 

 136, pi. 7, figs. 11a, 115. 



Description. — Carapace obliquely subovate, much narrower in 

 front than behind ; hinge straight, somewhat exceeding half the length 

 of carapace, the anterior angle fairly distinct, the anterior margin 

 obliquely subtruncate, extending a little beyond the angle, rounding 

 sharply below into the gently convex and anteriorly ascending ventral 

 margin; posterior end long and wide, strongly rounded in outline 

 except the upper third, which rises with a straight slope to the obtuse 

 but clearly defined postcardinal angle. Valves strongly convex, with 

 or without a very narrow, barely distinguishable rim ; gap rather wide 

 at both ends and along ventral side. Surface of shell polished and 

 punctated as usual. 



Dimensions of typical specimen: Greatest length (obliquely across 

 carapace) 4.1 mm., length of hinge line 2.8 mm., greatest height 2.8 

 mm., thickness about 2.1 mm. 



Remarks. — Matthew's collection of types contains three specimens. 

 One of these, an entire carapace, was figured by Matthew and is the 

 original of Figure 5 on Plate 9. It is, therefore, regarded as the 

 holotype. The other two specimens are somewhat imperfectly pre- 

 served valves, but, so far as can be seen, belong to the same species 

 as the type. 



Matthew's figure of the left side of the type represents the ventral 

 margin as more convex, and consequently the height of the posterior 

 half as somewhat greater than it is in the original. The dorsal angles 

 also are too much rounded in his illustration. These slight inaccura- 

 cies are corrected in Figure 6 on the same plate. 



The relatively narrow anterior extremity and the pronounced retral 

 swing of the outline serve to distinguish the species from /. lippa. In 

 I. lonqula, however, both of these features are developed to an even 

 greater extent. /. dermatoides is a close ally, and the discrimination 

 of the two forms may occasionally prove difficult. However, with 

 good specimens the nearly rectangular antero-dorsal part of the out- 

 line and the more convex ventral side of /. dermatoides make the task 

 of separation reasonably easy. 



Another very close alliance seems to be with I. anderssoni (Wiman) 

 from the Glauconite sandstone of Sweden. Wiman's type evidently 

 is somewhat crushed ; but if the outline as represented in the illustra- 



