AKT. 4 CAMBRIAN CONCHOSTRACA — ULRICH AND BASSLER 43 



The genus as now known to us embraces four or five typical species 

 and three or four varieties of these probably deserving separate names. 

 Besides these we refer here the two recognizable species of Escasona 

 Matthew, which we regard as congeneric. 



Matthew referred four species to his proposed genus Escasona. 

 One of these, E. ovata, he had previously described as a Beyrichona. 

 The type of the proposed genus is E. rutellum. Both of these species 

 differ from the more typical species of Beyrichona chiefly, if not 

 solely, in the contraction of their horizontal dimensions and in the 

 modification of the dorsal nodes and mesial depression incident to 

 this shortening of the valves. In our opinion, E. rutellum probably 

 and B. ovata certainly stand at one end, and B. triceps at the oppo- 

 site extreme of the development of a single generic type of which 

 Beyrichona tinea represents the typical or average expression. 



The types of Escasona ingens and E. 'vetus are so imperfect that it 

 is thought impossible to ever recognize other specimens of the species 

 with certainty. It would be to the advantage of science, therefore, 

 to drop these names, and with them Escasona, unless more complete 

 collections should prove the advisabiUty of retaining the name for 

 E. rutellum. 



BEYKICHONA PAPIUO Matthew 



Plate 6, Figure 11; Plate 7, Figures 5-7 



Beyrichona papilio Matthew, Trans. Royal Soc. Canada, vol. 3, sect. 4, 1886, 

 p. 65, pi. 6, figs. 20, 20o, 206; Trans. New York Acad. Sci., vol. 14, 1895, p. 

 134, pi. 7, figs. 4a-4c. 



Beyrichona triangula Matthew (part), Trans. New York Acad. Sci., vol. 14, 

 1895, p. 135, pi. 7, fig. 5. 



This, the genotype, is founded on a single but good specimen show- 

 ing two valves opened so that they lie in nearly the same plane. The 

 ventral angle is prominent, acute, the posterior edge abruptly trun- 

 cated, very slightly curved in outline and disposed at nearly a right 

 angle to the hinge line. The nodes are well developed, the posterior 

 one sharply ridged, differing thus from all the other species. 



As will be noted presently, two of the types of B. triangula (pi. 

 7, figs. 6, 7) are of this species. Judging from the Walcott collection, 

 the species is not common. 



Occurrence. — Lower Cambrian (Hanfordian, division Cl62-Cl64): 

 Hanford Brook, New Brunswick. 



Plesiotype.— Cat. No. 50501, U.S.N.M. (10c. 2i). 



BEYRICHONA TINEA Matthew 



Plate 6, Figures 1, 2, 5-7; Plate 7, Figure 4 



Beyrichona tinea Matthew, Trans. Royal Soc. Canada, vol. 3, sect. 4, 1886, 

 p. 66, pi. 6, figs. 21, 21o, 216; Trans. Royal Soc. Canada, vol. 11, sect. 4, 

 1894, p. 97; Trans. New York Acad. Sci., vol. 14, 1895, p. 134, pi. 7, figs. 

 6a-6c. 



