72 SYNOPSIS OF THE CHARACTERS OF THE 



CUCULL^EA ARGUTA. Phil. 

 Cucullsa arguta. Phil. Geol. York. 



^^7. Cli. — Rliomboidal, gibbous, twice as wide as long ; anterior end short, rounded ; posterior end obliquely 

 truncate, inferior angle acute ; a steep ridge separates the posterior slope from the body of the shell ; beaks pro- 

 minent ; surface with regular, transverse, imbricating sulci. 



This shell is so oblique, its posterior ridge so steep, and its angles so acute, that, when seen in the masses it 

 sometimes forms, it resembles an Avicula, and the great regularity of the sulci, which are parallel to the mar- 

 gin. Increases the resemblance ; the posterior end is more obhquely truncated than is usual in this genus, and 

 the ridge and angles sharper and more acute ; the beaks are small but prominent. Length five lines, width 

 ten lines. 



CuCULLiEA TENUISTRIA. M'Coy. (PL XII. fig. 10). 



Sp. Ch. — Subcylindrical, transversely oblong, rather more than twice as wide as long; evenly convex, 

 finely striated, obliquely from the beaks to the ventral margin ; beaks approximate, small, close to the anterior 

 end ; anterior side small, compressed, posterior slope not defined, compressed ; dorsal and ventral margins 

 parallel. 



Nearly twice and a half as wide as long ; valves evenly convex, with numerous fine, oblique, anterior striae ; 

 anterior side very short; no ridge defining the posterior slope, which passes gradually into the body of the shell. 

 Length one inch, width two inches five lines. 



Byssoaeca. Sw. 



Gen. Ch. — Shell transversely oblong, angulated, hinge-line straight, often eared, teeth small; valves 

 gaping at the basal margin, leaving a sinus when the valves are closed for the passage of the byssus. 



Byssoaeca clathrata. M'Coy. (PL XI. fig. 34). 



Sp. Ch. — Three times as wide as long, convex, cylindrical, anterior end obtuse, rectangular at the hinge- 

 line, posterior end rectangular, posterior slope separated from the body of the shell by an obscure ridge; entire 

 surface reticulated with equal, round, radiating stri», which are about their own diameter asunder, and trans- 

 verse striae, which are equal to the longitudinal in size and distance from each other. 



The peculiar character of this singular species consists in the longitudinal and transverse striae being ex- 

 actly equal in size and relative distance, the spaces produced by the crossing of those striae are square, at each 

 intersection a small tubercle is produced. Length three lines, width nine lines. 



Byssoaeca cosTELLATA. M'Coy. (PL XI. fig. 36). 



Sp. Ch. — Width twice and a half the length, very convex, beaks rather large, prominent, anterior end 

 short, pointed ; posterior end rectangular ; posterior slope separated from the body of the shell by a very steep 

 ridge ; entire surface ornamented with equal, rounded, radiating ribs. 



This well-marked species is remarkable for the regular equality of its radiating ribs, which, together with 

 its great width, will distinguish it from all other Palffiozoic species; the sinus in the front is very small. Length 

 four lines, width eleven lines. 



Byssoaeca Lanceolata. M'Coy. (PL XL fig. 33). 



Sp. Ch. — Transversely trigonal; width four and a half times the length; hinge-line rather less than two- 

 thirds the length ; anterior end very broad, obliquely truncated, smooth, concave, separated from the body of 

 the shell by a strong, curved ridge from the beak ; posterior end very narrow, the extremity obliquely trun- 



