62 CATALOGUE OF 



outwardly directed ; last fairly strong ; apex near 

 distal end, prominent, pointed, obtuse; ridges sharp, 

 umbonal sloping gradually, distal very steeply. 



Dimensions. A specimen from Wai worth Wood (Londonderry) 

 from the collection of Mr. J. N. Milne measures : Long. 4, Alt. 3-4, 

 Crass. 2'3 mm. ; from Comber (Down) from the same collection is 

 one 4 x 3-2 x 2-2 mm. 



Jenyns distinguished two varieties : ft in which the umbones 

 were more prominent, and y in which the striie were more pro- 

 nounced ; these, however, are quite immaterial distinctions in the 

 light of present knowledge. His remark that "the shell is certainly 

 somewhat intermediate in form between that of Cyclcts [Sphariwn] 

 and Pisidium " is very apt, the comparative great length of the 

 laterals a.m. and p. ///. as well as the shape recalling &plt<i'i-i.ii<, 

 and usually serving to separate it from the other species of 

 Pisidium. 



It may further be distinguished fiom P. personatum, which 

 it frequently, and from P. nit'ulum, which it sometimes, closely 

 resembles in external form, by the hinge-characters. The hinge- 

 plate is not so broad, the teeth are more slender, and the cardinals 

 straighter and more parallel to the hin<;e-line than in either 

 of those species. It of course lacks the callus peculiar to P. per- 

 sonatum, and is more equivalve and oval than P. nitidum. 



The periostracum in perishing or half-dead shells is usually 

 iridescent, an appearance less commonly met with in other species, 

 and in shell-marls the umbones are very often broken away. The 

 transition from the nepionic to the adult shell is frequently, .is in 

 P. nitidum, marked by a series of closer set, strongly marked striae, 

 which are, however, both coarser and more wide apart than in the 

 latter species. 



Of the numerous forms and variants figured, those from Tenby, 

 Pembrokeshire (PL XXI, f. 13 & 33 a, 6) are perhaps the most 

 typical. The rounded form outwardly resembling P. personatum 

 is represented from llhosncigr, Anglesey (PL XXI, f. 19), and from 

 the Botanic Gardens at Naples (PL XXI, f. 29 a-d} ; whilst the 

 extremely elongate-oval is shown from Comber, Co. Down (PL XXI, 

 f. 14 & 38 a, b\ and the lliver Po (PL XXI, f. 16 & 34 o, b). 

 An approach to the trigonal form of P. casertannm is that from 

 Walworth Wood, Co. Londonderry (PL XXI, f. 39 a, b). 



In degree of inflation this species varies greatly : the example 

 from Woodburn Dam, Co. Antrim (PI. XXI, f. 6) is about normal. 

 The extreme compression appears in specimens from Bracebridge, 

 Warwick (PL XXI, f. 4) and Kilcorrig, Co. Antrim (PI. XXI, f. 5), 

 whilst an abnormally tumid example is shown from Comber, Co. 

 Down (PL XXI, f. 8). 



A case of abnormal dentition was met with in an individual from 

 Walworth Wood, Co. Londonderry (PL XXI, f. 35 a, b), the left 

 valve having two anterior lateral teeth. 



