36 The Irish Naturalist. March, 



The systematic position of the present species is, hke 

 that of most of our Pisidia, not easy to determine. In 

 striation its shell comes between P. ohtiisale and P. nitidum ;i 

 the siphonal tube of the animal resembles that of P. 

 ohtiisale, but not that of P. nitidum ; the general hinge 

 characters of the shell are more nearly allied to P. nitidum, 

 though in the case of depauperate shells they may simulate 

 those of some forms of P. ohtiisale. The fry of both P. nitidum 

 and P. ohtiisale are as a rule distinctly ovate and have a 

 longer and straighter hinge-line than those of P. hiherniciim, 

 which in outline are markedly quadrate. If we select any 

 one of these characters its aihnities can readily be fixed, 

 but when all are considered in conjunction the problem 

 becomes difficult. P. hibernicum is generally most difficult 

 to separate from small tumid forms of P. nitidum, though 

 when the two species are taken in association they will not 

 often be confounded, as their general appearance, even if 

 the siphons cannot be examined, should prove sufficient to 

 separate them. It may also at times resemble forms of 

 P. ohtiisale or P. milium, and has even been mistaken for 

 P. personatum and P. lilljehorgi. In this connection it is 

 interesting to state that the specimens of Jenyns' P. 

 ohtusale, var. (^, preserved at Bath are, in Mr. Oldham's 

 opinion referable to P. hihernicum. Jenyns' diagnosis of 

 this variety well fits normal forms of P. hihernicum, and his 

 reference to its ochraceous colour is decidedly appropriate 

 since the animals of most living examples which we have 

 examined were either deep yellow, pinkish-yellow or rose- 

 pink. This coloration can as a rule be seen distinctly, even 

 when the shells are exteriorly encrusted. The following 

 table of characters may be useful to other students of the 

 group, when taken in conjunction with the figures on the 

 accompanying plates. 



p. hibernicum Westerlund : — Shell — equilateral. Umbones — small, 

 prominent. Fry — squarrose. Striation — regular, close and well 

 marked. Siphon — narrow, margin simple (Pi. I., fig. 2a.). Liga- 



1 P. nitidum of Jenyns, which probably ecpials the P. pusiUum of 

 Mr. B. B. Woodward's Cat., as pointed out by A. W. S. in jouryi. of Conch., 

 vol. XV.. pp. 235-239, 1918, 



