STKl.KDX : IMSIDIUM FAUNA Ol' THE CRAND JUNCTION CANAL. 297 



usually crosses the unihonal end of rr. IP (this ridge is complementary 

 to a similar feature in the right valve, which originates along the 

 inner margin of the hinge-plate under c. j and dies away into the 

 shell wall above the upper end of a. Ill); a. II and /. // 

 long, straight, and stoutly built, with the cones fairly prominent but 

 rounded ; ligament pit comparatively large, bounded below by a faint, 

 somewhat sinuous ridge ; at its maximum breadth the ligament pit 

 is almost the full width of the hinge-plate ; between the ligament pit 

 and the cardinal teeth, in each valve, there is a slight hollow cutting 

 into the hinge-plate. 



¥k\' oval, almost smooth, about '5 x '3 mm., and not so compressed 

 as are the young of most species of the genus. 



Habitat : — In the Grand Junction Canal, near Marsworth Church, 

 Bucks., and other places. Considering that F. parvi/lum has now 

 proved to have quite a wide range in the south midlands of England, 

 though overlooked until 1916, it is not surprising that this small 

 shell has also been passed by, since it would appear to be less com- 

 mon and not so widely distributed in these islands. The first speci- 

 mens to be detected were three sent by Mr. Oldham amongst a set 

 of P. parvuhim from the canal at Cheddington. They were easily 

 picked out from that species by their beautiful striation, and were 

 laid aside as probably the young of some other inhabitant of the 

 canal ; but an examination of the other species at once showed that 

 this was not the case, and among further specimens collected by Mr. 

 Oldham it was observed that the larger and older shells were eroded 

 like the fully-grown examples of the other Pisidia. In 19 16 I was 

 satisfied that Mr. Oldham's second gathering included mature speci- 

 mens and that it was a distinct species. In the autumn of 191 7 the 

 former conclusion was confirmed, as on boiling a medium sized 

 specimen, about i"6 inm. in length, I obtained from it two fry. As I 

 have been unable to find a description of any known species which 

 fits it, I have decided to name it by the character that first drew my 

 attention to it — namely, its wonderfully regular and line-like striee. 



Systematically it is difficult to place. Exteriorly it is not unlike 

 the young of the tumid forms of P. subtruncatuni that occur in the 

 canal, though interiorly it more nearly resembles P. parvulum in its 

 hinge characters. P. tetiuilineaitim may be distinguished from young, 

 thick-shelled P. subtrujiratufn by its more regular and strongly-marked 

 striation, smaller and more curved cardinal teeth, and by its lateral 

 teeth being almost equi-distant from the cardinals — in P. subtruncatum 

 the anterior teeth are much nearer to the cardinals than the posterior 



I If a thin-shelled form of P. tcnuilineatum exists it is unlikely that this feature will be found 

 In it, since similar ridges occur in thickened forms of utlier species, though not appearing in 

 normal shells uf the same species. 



