PATEKINID^. 345 



I received from Whiteaves what was supposed to be the type specimen of the ventral valve, 

 from Trois Pistoles, but I find that it does not belong to the species described by Billings 

 [1872b, p. 477]. The area is strongly marked, but the pseudodeltidium in the specimen before 

 me is very short, not over one-fifth the height of the deltidial area. In this respect it resembles 

 the pseudodeltidium of Micromitra (Paterina) crenistria (Walcott). It is described in tlais 

 monograph as M. (P.) logani (Walcott). Under date of July 23, 1896, Professor Whiteaves 

 wrote that there were no other specimens or electrotypes of the types of 31. (P.) hella (Billings) 

 in the collections of the Geological Survey of Canada, nor of the closely allied species of the 

 same genus from Topsail Head, mentioned by Billings [1872b, p. 478]. (See pp. 335 and 350, 

 and PL II, figs. 1 and Ic.) 



Billings [1872b, pp. 477-478] describes the genus and species as follows: 



The ventral 7 valve of /. hella is conical, strongly elevated at the beak, hinge line nearly straight, posterior angles 

 narrowly rounded, sides and front nearly uniformly rounded, forming rather more than a semicircle. Posterior side 

 with a large false area and a convex pseudodeltidium, the width of which at the hinge line is nearly one-third the 

 whole width of the shell. The dorsal valve is semicircular, moderately convex, most elevated at the beak. The hinge 

 line appears to be straight. The form and structure of the posterior side (such as the area, foramen, deltidium, etc.) 

 can not be made out from the specimen, owing to its imperfection. The surface is covered with fine concentric striae, 

 which in the ventral? valve are continued around on the area. Of these striae there appear to be from fifteen to twenty 

 in the width of one line, their size varying somewhat in different parts of the specimen. There are also a few obscure 

 radiating striae. Width of ventral valve, seven lines; length, five lines; height, two lines. 



Very little is to be added to this description from a study of the material from the north 

 side of the Straits of Belleisle and from Pennsylvania, except that the examination of a num- 

 ber of specimens has revealed the fact that there is no true perforation in the beak. Billings 

 states that in the specimen described by him there is an aperture in the beak, but in another 

 there is no appearance of one. I observed a narrow furrow near the apex of the beak in 

 a specimen from L'Anse au Loup, Labrador, but there was no aperture through the shell. 

 Several other specimens afford no traces of even a slight slit in front of the beak. The presence 

 of this depression or furrow is mentioned, and its character set forth in the description of the 

 genus Micromitra (p. 335). 



Formation and locality. — Lower Cambrian : (2n) Limestone bowlders in conglomerate, along the shore of the St. 

 Lawrence, near Trois Pistoles, Temiscouata County; and (2o) limestone bowlders in conglomerate on shore at east entrance 

 to harbor at Bic, Rimouski County; both in Quebec, Canada. 



(314e [Billings, 1872b, p. 478]) Limestone at Topsail Head, Conception Bay, Newfoundland. 



(393a) Limestone [Walcott, 1891b, p. 254] at L'Anse au Loup, on the north shore of the Straits of Belleisle, 

 Labrador. 



(326g [Grabau, 1900, p. 617]) Limestones at East Point, Nahant, Essex County, Massachusetts. 



(49) Sandstone on Codorus Creek, 0.125 mile (0.2 km.) below Meyer's mill, near Emigsville, York County; 

 (346a) limestones in Landis Valley, Lancaster County; and (49w) limestones in raih-oad cut 0.25 mile (0.4 km.) south 

 of Emigsville, York County; all in Pennsylvania. 



(25) Sandstone just above Parker's quarry, near Georgia, Franklin County, Vermont. 



/ Micromitra (Paterina) crenistria (Walcott). 

 Plate III, figures 4, 4a-b. 



Iphidea crenistria Walcott, 1897, Proc. U. S. Nat. Mus., vol. 19, p. 713, PI. LIX, figs. 4, 4a-b. (Described and discussed 

 essentially as below as a new species. The specimen represented by figs. 4, 4a-b is redrawn in this monogi-aph, 

 PI. Ill, figs. 4, 4a-b.) 



Ventral valve subconical, beak nearly apical, curving slightly over to the pseudodeltidium. 

 False area very narrow, separated from the curvature of the shell by being turned somewhat 

 abruptly backward. Pseudodeltidium comparatively broad, but decidedly short as compared 

 with that of Micromitra (Paterina) hella (Billings) or M. (P.) swperia (Walcott). It is more 

 the type of that of M. (P.) labradorica (Billings). It is strongly arched, leaving a broad, high 

 space between it and the plane of the shell. 



Dorsal valve unknown. Surface of ventral valve marked by very fine, slightly crenulated 

 striae that are so crowded on the false area that they are nearly lost, and only one or two varices 

 of growth and a few strise are shown on the pseudodeltidium. No traces of an apical opening 

 have been seen. On some shells a faint furrow is seen on the apex. 



