120 CATALOGUE OF 



naturalists had before them, lioth Pfeiffer and Jenyns question 

 whether their specimens belonged to Lamarck's species ; but while 

 Pfeiffer's diagnosis and figures are loo vague io permit of an 

 authoritative statement as to the identity of his species, there can 

 be no question concerning Jenyns', as a glance at his specimens at 

 Bath shows. Jeffreys, as already mentioned (p. 11), had confused 

 Jenyns'' P. nitidum and the present species. 



Under the circumstances it may be well to give tho three 

 successive descriptions. Lamarck's (92, v, p. 559) is: 



" C. testa ovali, tumida, subinrcquilatera, pellucida, fragilis- 

 sima ; umbone obtusissimo. Mon cabinet .... Elle a des 

 rapports avec la suivante [Cyclas fontinalis]." 



PfeifFer's diagnosis (134, i, p. 125) is more extended : 



" P. testa oblique cordata, ventricosa, tenuissime striata, 

 pellucida, fragilissima ; umbone obtusissimo .... GeJtaus : 

 schief herzformig, bauehig, etwas ungleichseitig, gliin/.end, 

 sehr fein kuum bemeiklich gestreift, dun-hsichtig, gelblich 

 weiss. Her untere Hand scharf. Die Wirbelspitzen. vorste- 

 liend, sehr stiunpf, gerundet." 



Jenyns (83. p. -301), while assigning to coloration more than its 

 due, gives the best summary of the external characters : 



"P. testa globosa, oblique subovali, tenuissime striata; 

 umbonibus prominulis, obtusissirnis .......... 



Var. ft. Test a ovato-trigona, ventricosissima, margine obtu- 

 sissimo. ....... Testa globoso-ovalis, vcntricosissima, 



crassitudine fere altitudinem a?quanti, nitida, subtiliter 

 striata ; plerumque virescenti-nigra vel ochraceo-nigricans, 

 /ona marginal! (junioribus latissima) lutescenti, interdum 

 suhaimmtia ; rarius omnino lutescens : umbones tumidi, 

 obtuse rotundati, paulo promincntes. 



Var. ft. gaudet test.a ventricosiori, margine basali obtu- 

 sissimo, quo minuatur altitude, et forma magis trigona vcl 

 ovato-trigona proven it. Ha?c vaiietas plerumque nigricans, 

 ochraceo plus minusve fucata/' 



To this it is only necessary to add a detailed description of the 

 hinge characters: 



(PI. II, f. 10 ; IV, f. 4) about | the length of the shell, 

 strong, narrow, especially at the nmbo, projecting well inwards, 

 roundly arched, with slight anterior flexure in right valve. 



R.V. a. /. about -/^ the length of the hinge-line, strong, stout, 

 inclined a little inwards ; lase strong and swollen 

 (especially in old specimens) ; apex central, or on 

 umbonal side of centre, prominent, acuminate to 

 rounded obtuse; r'ulyes rounded, sloping about equally, 

 fairly steeply (in old specimens fairly gradually), 



