PtTNCTURELLA. 233 



This species in form somewhat resembles P. agger, but is longer 

 and narrower ; the sculpture and slit are very markedly, and the 

 apex unmistakeably,- different. In all these respects it differs from 

 P. noachina, to the young of which it has a vague resemblance. 

 That species also is usually in all stages of growth narrower in 

 front than behind. I failed to recognize either the white or trans- 

 parent specks one or other of which are generally present in the 

 Fissurellidce. ( Wats.) 



North Atlantic; Culebra Id., W. Indies, 390 fms. 



P. dathrata JEFFR. P. Z. S. 1882, p. 676, t. 1, f. 11. WATSON 

 Challenger Gastrop., p. 39. 



P. AGGER Watson. PI. 26, figs. 32, 33, 34, 35. 



Shell small, porcellaneous, oval, broader in front ; sides slopes 

 straight, the front convex, the back concave rather high, but with 

 the top depressed and thrust out backwards, yet little projecting, the 

 apex being flattened out on the posterior slope ; there are many not 

 crowded slight radiating ribs set with stumpy prickles ; the long and 

 lanceolate slit is on the crest, and from it a ridge runs down the front 

 slope. Sculpture : The riblets are very slight, but are made distinct 

 by the little triangular prickles which pretty closely stud them ; 

 these prickles more irregularly and remotely tally with feeble lines 

 of growth ; the ridge down the front slope is small, and is crowned 

 with two of the riblets parted by a narrow furrow ; these riblets 

 along the slit rise into sharp laminae Color : The shell is porcel- 

 lanous white, with a slight ruddy tinge. Apex somewhat depressed 

 and shortly projected backward, curling in on the central line of the 

 shell, but with the extreme tip flattened out on the posterior right 

 slope. There are 21 whorls in all. Slit lies high on the front slope, 

 distant from the apex about once, and from the margin about twice 

 its own length. It is long and narrow, bluntly rounded at the 

 upper end, and produced in front to a long sharp point. Margin is 

 thin and not fretted with the rib ends. Inside glassy, deeply 

 hollowed into the apex, feebly rayed, and having the rays picked 

 out with bright specks corresponding to the eternal prickles ; a long 

 shallow furrow, bordered by a minute ridge on either side, runs 

 widening upwards from the margin to the slit, which is covered in 

 all its length by the somewhat contracted, sinuous edged, cross 

 scored, straight, glassy septum, which arches in to the apex. 



Length O17, breadth O13, alt. 0'09, inch. . (Wat*.*) 



