PERICYCLUS TRAPEZOIDALIS. 145 



tlie edges of all the inner whorls, about three-eighths of the diameter' of the shell 

 in width, with subangular margin and nearly vertical sides. Whorl subtrapezoidal 

 in cross-section, about as high as wide ; indented to about one-fourth of its height 

 by the preceding whorl ; periphery narrowly convex, imperfectly defined ; sides 

 feebly convex, a little flattened near the um1)ilicus, and becoming more flattened 

 and convergent on the body-chamber ; umbilical zone well defined, narrow, almost 

 perpendicular to the plane of symmetry of the shell. Body-chamber occupying 

 uearljr a complete whorl ; aperture not seen, but the peristome (judging by the 

 lines of growth) probably with a broad, feeble lateral crest and a fairly deep hypo- 

 nomic sinus. Depth of chambers not seen; suture-line only imperfectly seen. 

 Test ornamented with narrow, prominent ribs, which pass obliquely backward from 

 the umbilical margin, cross the lateral area in a feeble anteriorly convex curve, and 

 form on the periphery a fairly deep and wide hyponomic sinus ; interspaces flat, 

 nearly twice as Avide as the ribs ; the whole surface of the ribs and interspaces 

 (Avhen well preserved) with fine, close-set lines of growth, especially on the body- 

 chamber. The outer whorl with numerous (nine or ten) constrictions, following 

 the course of the ornaments of the test. ' Wrinkle-layer ' composed of fine, regu- 

 lar, close-set longitudinal lines." 



I)uiU'iisio)is. 



Diameter of shell .... 

 „ umbilicus (suture to suture) 



(edge to edge) 

 Height of outer whorl 



„ al)ove preceding Avliorl (about) . 

 Thickness of outer whorl 



Supplementary Description. — Crossing the ribs and interspaces nearly at a right 

 angle, and thus running in accordance with the curvature of the whorls, are a series 

 of fine raised lines which may easily be seen in a favourable light by the naked 

 eye. On examining the surface of the test with a low magnifying power there is 

 a decided appearance of these lines having formed nodes in crossing the ribs, but 

 the condition of preservation makes this point a doubtful one (PI. XXXIX, fig. 2 c). 



The highly ornate character of the test in this species will make it easily recog- 

 nisable should other specimens fall into the hands of collectors. 



Owing partly to distortion, partly perhaps to the grinding of the surface in tlu' 

 effort to develop the sutures having been carried too deep, the latter are not satis- 

 factorily shown, but their general plan can very well be seen (PI. XXXIX, fig. 2 d). 



Uniqiie specimen in 



Museum of Science 



and Art, Dublin. 



141 



mm. 



49 





53 





51 





30 





50 





