SPECIES OF PISIDITJM. 117 



ubique sulcato-striata, margin! bus omnibus regulariter arcu- 

 atis, parle posteriore brevissima, unteriore duplo longiore, 

 ad raargines comprcssa, sed obtusissima ; umboues validi, lati, 

 tumido-rotundati, promiuentes." 



No details were given concerning the hinge characters, which 

 may be thus particularized : 



Hinge (PI. II, f. 9 ; IV, f. 5) about f the length of the shell, 

 narrow, scarcely projecting save at the umbo, not very strong, 

 flatly arcuate. 



11. V. a. I. about | the length of the hinge-line, narrow, not very 

 strong, strongly bowed inwards ; base fairly strong, 

 somewhat swollen ; apex central, prominent, pointed 

 obtuse ; ridyes somewhat rounded, sloping about 

 equally, at first steeply and then running out 

 horizontally. 



a. ///. about ^ the length of a. /., from which it is separated 

 by a deep fossa, not very strong or prominent ; apex 

 about central, not very prominent, somewhat out- 

 wardly inclined, rounded obtuse ; ridyes sloping away 

 about equally. 



3. lamelliform, prominent, nearly straight, top irregular; 

 b somewhat thickened, sometimes sulcate. 



p. /. about ^ the length of the hinge-line, narrow, fairly 

 strong, straight, erect ; base fairly strong and swollen ; 

 apex distal side of centre, prominent, somewhat 

 acuminate; ridges fairly sharp, umbonal descending 

 fairly steeply at first, then gradually, distal very 

 steep. 



p. in. about ^ the length of p. /., from which it is separated 

 by a deep fossa, very inconspicuous, closely apposed to 

 the shell-wall ; apex very obtuse. 



L.V. , //. about | the length of the hinge-line, rather narrow, 

 strong, straight ; base strong, somewhat swollen ; apex 

 central, exceedingly prominent, acuminate, somewhat 

 distally directed, with peculiar outward twist ; ridyes 

 sharp, distal sloping down very steeply and descending 

 to a lower level than the umbonal, which is not quite 

 so steep. 



2. very prominent, obtusely triangular, with backwardly 

 directed apex, base continuous with that of a. //. 



4. lamelliform, not prominent, nearly straight, running 

 backwards from above 2 with slight downward diagonal 

 inclination. 



y>. u. about | the length of the hinge-line, rather narrow, 

 strong, straight ; base strong, slightly swollen ; apex 

 near distal end, very prominent, acuminate ; ridges 

 sharp, descending about equally, very steeply at first 

 and then gradually. 



