SPECIES OF riSIDIUM. 45 



"P. testa orbiculato-ovali, nilidissima, tenuiter striata; 

 uiubonibus obtusiusculis, striis paucis profundioribus. 



" Testa minime variabilis, orbiculato-ovalis, parum inaequi- 

 latcralis; pra3Cedenti [P. pusil'luin'] paulo convexior, et pro 

 ratione longitudinis altior ; aibo lutescens, nitidissima, raro 

 aut nunquam sorde aut rubigine obtecta, tenuiter striata, 

 striis hie iliic, proecipue 3-5 umbones transeuntibus, dis- 

 tinctius incisis : umbones obtusiusculi, dorsalem marginem 

 paulb superantes." 



The hinge-characters are as follows : 



Hinne (PL I, f. 9 ; III, f. 6) a little more than j the length 

 of the shell, strong, fairly wide, projecting well inwards, arcuate 

 with slight anterior flexure. 



li.V. a. I. about 3 the length of the hinge-line, narrow, strong, 

 bowed and inclined inwards; base very strong and 

 swollen ; apex central, fairly prominent, pointed 

 obtuse ; ridges fairly rounded, sloping fairly equally, 

 steeply at first, then giadually. 



a. in. about i the length of a. /.', the whole forming a narrow, 

 arched prominence, somewhat apposed to the shell- 

 margin. 



3. strong, very prominent, Granulate top, sharply flexed ; 

 a narrow, approximately parallel to the shell-margin ; 

 b thicker, strongly sulcate, diagonally directed across 

 the hinge-plate to near its inner margin. 

 p. f. about | the length of the hinge-line, narrow, strong, 

 curving somewhat with the hinge-margin, erect; 

 base strong, somewhat swollen; apex distal side of 

 centre, not very prominent, obtuse-pointed; ridges 

 sharp, sloping almost equally and fairly steeply. 

 p. ///. about the length of, and parallel to p. /., narrow, 

 erect ; apex central, prominent, obtuse-pointed ; ridyes 

 compressed, sloping about equally. 



L.V. a. II. about -J the length of the hinge-line, narrow, strong, 

 nearly straight ; base strong and slightly swollen ; apex 

 central, very prominent, acuminate ; ridyes fairly 

 sharp, distal at first exceedingly steep, then nearly 

 horizontal, umbonal sloping very steeply and descending 

 to below the level of the distal. 



0. very prominent, obtusely triangular, fairly parallel 

 with the inner margin of the hinge-plate, base con- 

 tinuous wiih that of a. //., pointed apex directed 

 slightly backwards. 



4 very prominent, thin, sharp, flat-topped, at first fairly 

 parallel to the shell-margin, then curving backwards 

 round , and almost reaching the inner margin of the 

 hinge-plate. 



p. n. about ^ the length of the hinge-line, narrow, strong, 

 erect, straight ; base strong, scarcely swollen ; apex. 



