ART. 4 CAMBRIAN CONCHOSTRACA — ULRICH AND BASSLER 93 



in specimens of otherwise very different Cambrian species studied in 

 the course of this work. 



Occurrence. — Lower Cambrian (Hanfordian, division Cl63). Mat- 

 thew's type is from Hanford Brook, St. John County, New Brunswick. 

 Specimens in the United States National Museum are from the same 

 locality and probably from the same beds. A single example (E36), 

 doubtfully referred to the species, came from the Etcheminian at 

 Dugald Brook, Cape Breton, Nova Scotia. 



Plesiotype.— Cat. No. 56499, U.S.N.M. 



Family LIMNADIIDAE Baird 



Genus LEPIDITTA Matthew 



Lepiditta Matthew, Trans. Royal Soc. Canada, vol. 3, sec. 4, 1886, p. 61. — 

 Miller, North Amer. Geol. and Pal., 1889, p. 553.— Matthew, Trans. 

 New York Acad. Sci., vol. 15, 1896, p. 194. 



Description. — Carapace small, a millimeter or two in length 

 bivalved, obliquely semielliptical or semicircular, narrow and more 

 or less acuminate posteriorly, wider and nearly rectangular anteriorly, 

 with the hinge straight and extending the full length of the valves. 

 Surface moderately convex, usually with a low, conical, umbonelike 

 elevation near or in front of the middle of the dorsal edge. Test thin, 

 composition doubtful, probably calcareous; externally marked with 

 concentric lines or undulations and sometimes with minute radial 

 lines rarely smooth. Casts of the interior exhibit a short, vertical 

 fissure or depression suggesting a clavicle on inner side of shell. 



Genotype. — Lepiditta alata Matthew. Middle Cambrian. 



Remarks. — The systematic position of this genus is somewhat doubt- 

 ful. The shell being usually dissolved away in the shales, it is thought 

 likely to have been more calcareous than in the great majority of 

 Cambrian bivalved Crustacea. In this respect then it would appear 

 to agree with Fordilla in which the shell is known to be highly cal- 

 careous. It agrees further with that genus in presenting an aspect 

 simulating that of a minute pelecypod. These peculiarities suggest 

 the phyllopod genus Estheria more than any other bivalved crustacean 

 known, and the resemblance is believed to be sufficient to warrant the 

 provisional reference of these two Cambrian genera to the Limnadii- 

 dae. The principal objection to this arrangement lies in the fact that 

 Crustacea like Estheria are almost if not entirely unknown in rocks 

 older than Devonian. A possible exception is Orthonotella faberi 

 Miller, described as a pelecypod, which is found in the upper part 

 of the Ordovician at Cincinnati, Ohio. Other possible exceptions are 

 Ischyrina and Technophorus , both founded on Ordovician species 

 hitherto referred to the Pelecypoda. 



The separated valves of the Middle Devonian genus Schizodiscus 

 Clarke agree so closely with those of Lepiditta that it is really difficult 



