120 CATALOGUE OF 



naturalists had before them. Both Pfeiffer and Jenyns question 

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

 Pfeiffer's diagnosis and figures are too vague to permit of an 

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

 be no question concerning Jeuyns', 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 the three 

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



" C. testa ovali, tumida, subina3quilatera, pellucida, fragilis- 

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

 rapports avec la suivante \Gydas fontinalis]" 



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



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

 pellucida, fragilissima ; nmbone obtusissimo .... Gehiius : 

 schief herzformig, bauchig, etwas ungleichseitig, glanzend, 

 sehr fein kaum bemerklich gestreift, dtm-bsichtig, gelblich 

 weiss. Der untere Hand scharf. Die Wirbelspitzen vorste- 

 heud, sehr stumpf, 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 stria! a; 



umbonibus prominulis, obtnsissimis 



Var. ft. Testa ovato-trigona, ventricosissima, margine obtu- 

 sissimo Testa globoso-ovalis, ventricosissima, 



crassitudine fere altitudinem asquanti, nitida, subtiliter 

 striata ; plerumque virescenti-nigra vel ochraceo-nigricans, 

 zona marginali (junioribus latissima) lutescenti, interdum 

 subaurantia ; rarius omnino lutescens : umbones tumidi, 

 obtuse rotundaii, paulo prominentes. 



Var. /?. gaudet testa ventricosiori, margine basali obtu- 

 sissimo, quo minuatur altitudo, et forma magis trigona vcl 

 ovato-trigona proven it. Ha?c varietas plerumque nigrieans, 

 ochraceo plus minusve fucata." 



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

 hinge characters : 



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

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

 roundly arched, with slight anterior flexure in right valve. 



H.V. a. /. about T 5 ^ 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; ridqes rounded, sloping about equally, 

 fairly steeply (iu old specimens fairly gradually). 



