SPECIES OF PISIDIUM. 85 



1875. ? Pisidium tumidum, Colbeau : Clessin in Kiistei 1 , Syst. Conch.- 

 Cab. ix, abth. 3, Cycladeen, p. 47, pi. v, f. 13-15. 



1878. Pisidium poulseni, n. sp. : Clessiu, Malakozool. Blatt. 1878, p. 124, 

 pi. v, f. 6. 



1898. ? Pisidium costulatum, n. sp. : Westeilund, Ann. Mus. Zool. Acad. 

 imp. Sci. St. Petersb. 1898, p. 180. 



Although as already mentioned examples of this well-marked foim 

 existed in Jenyns' collection, he unfortunately did not detect its 

 specific distinctness, nor at a later date did Jeffreys, though there is 

 reason to believe that it formed one element, and perhaps the prin- 

 cipal one, of his composite P. fontinale. Its discovery in Britain 

 was due to Dr. Johansen who pointed it out to me iu 1901, and it 

 was shortly afterwards placed on record (88, p. 17). 



Malm's diagnosis (106, p. 92) is : 



" C. peroblique ovalis, veniricosa, tenuissime striata, niti- 

 dula ; albida, zona inframediana fusca plerumque ornata ; 

 natibus convcxis, umbonibus parum prominulis. Sipholongus 

 gracilis, subconicus, truncatus." 



He further adds : [Translation] 



" This is ... easily recognized by its oblique shell, which 

 to the naked eye appears smooth. Its shape is a slightly 

 compressed oviform. Shell thin and fragile. Curve of dorsal 

 margin rather regular, though perceptibly straightened ante- 

 riorly where it merges into the somewhat narrowly rounded 

 anterior margin. Curve of the ventral margin less convex 

 than that of the cardinal but more sharply ascending towards 

 the anterior as well as towards the posterior margin, which 

 is also slightly convex, and in certain individuals presents 

 inferiorly a not inconspicuous angle. There is a slightly 

 perceptible obtuse angle where the dorsal margin passes into 

 the somewhat rounded, steeply descending, subtruncate posterior 

 margin." 



Save for the absence of appendiculse on the umbones it closely 

 resembles P. henslowanum in external appearance. The hinge, 

 however, differs in many respects as the following details show : 



Hinge (PI. II, f. 3 ; III, f. 7) about f the length of the shell, 

 strong, fairly wide, projecting well inwards, arcuate, more 

 sharply curved posteriorly. 



R.V. a. L about \ the length of the hinge-line, narrow, strong, 

 straight, inclined inwards ; base very strong and 

 swollen ; apex nearly central, prominent, blunt- 

 pointed ; ridges somewhat sharp, distal descending at 

 first very steeply, afterwards almost horizontal, um- 

 bonal sloping steeply to a lower level than the distal. 

 a.m. nearly g the length of a. /., narrow, apposed to the 

 shell-margin ; apex near the distal end, prominent, 



