42 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 78 



WALCOTTELLA LONGULA, new species 



Plate 5, Figure 7 



This species is so much Hke the preceding W. subtruncata that a 

 detailed description is unnecessary. The types of the two species 

 agree in every essential respect except that W. longula is relatively 

 more elongate, and that the tubercle is situated above instead of on 

 the level of the middle of the valve. None of the other species of 

 the genus is sufficiently Uke it to require comparisons. 



Dimensions: Greatest length 5.0 mm., length of hinge 3.6 mm., 

 greatest height 3.0 mm., thickness about 1.2 mm. 



Occurrence. — Same as W. subtruncata. 



Holotype.— Cat. No. 56491, U.S.N.M. 



BEYRICHONIDAE, new family 



Carapace bivalved with valves apparently fitting close together all 

 around the edges, leaving no gap. Surface of valves with a depressed 

 anterodorsal region bordered by variously arranged ridges or nodes. 

 Shell structure black, calcareo-corneous. 



Genus BEYRICHONA Matthew 



Beryrichona Matthew, Trans. Royal Soc. Canada, vol. 3, sect. 4, 1886, pp. 64, 



65. — Miller, North Amer. Geol. and Pal., 1889, p. 535. — Matthew, Trans. 



New York Acad. Sci., vol. 14, 1895, p. 133. — Jones, Geol. Mag., dec. 4, 



vol. 9, 1902, p. 402. 

 Escasona Matthew, Canadian Rec. Sci., vol. 8, 1902, p. 457. — Jones, Geol. 



Mag., dec. 4, vol. 9, 1902, p. 402. — Matthew, Geol. Surv. Canada, Rept. 



Cambrian Rocks Cape Breton, 1903, p. 167. (Genotype: E. rutellum 



Matthew.) 



The essential characters of the genus as founded on Beyrichona 

 papilio and B. tinea are (1) the subtriangular outline of the valves, 

 and (2) a broad, undefined depression in the dorsal slope. This de- 

 pression is limited in front by a short, nodelike ridge paralleling and 

 just within the upper third of the anterior border, and on the posterior 

 side by a thinner and more angular ridge starting at the hinge near 

 its posterior extremity and curving shghtly forward to its terminus 

 about the middle of each valve. The central part of the depressed 

 space is more or less convex, the swelling being most pronounced and 

 best defined in those species in which the relative height of the valves 

 is least (as B. triceps). In species like B. ovata, and B. rutellum, in 

 which the height reaches the opposite extreme, the mesial swelling is 

 entirely obsolete. In the most typical species, a low ridge, growing 

 obsolete posteriorly, begins at the antero-cardinal node and forms an 

 irregular swelling along the cardinal edge. Usually a very narrow 

 flattened border is developed along the free margins of the valves. 

 Length of valves in various species 3 to 5 mm. 



