Virfo7-i<in /'Wsi/.v, r,irl /A'. 219 



of the valves indicate a narrow body, with an elevated posterior 

 valve and a posteriorly situated mucro. In view of the fact that 

 Acanthochites, subgenus Notoplax, is distinguished by the numer- 

 ous slits in the axticulamentuin of the tail-valve, which latter 

 also projects beyond the integumentuui posteriorly, it is difti- 

 cult to discern the ground upon which the original authors of 

 this species founded their conclusions as to the genus in which 

 it should be placed, seeing that they record only median valves. 



Description of Posterior Valve. — Dimensions — Specimen a. 

 [4843] : Lengtli, 7 mm.; greatest width, 7 mm. Specimen b. 

 [4842] : Length, 8.5 mm.; greatest width, 9 mm. Distance 

 from point of raucro to external posterior border, 2.5 mm. 

 Height at anterior margin, 3.75 mm. (specimen a). Width of 

 sinus (spec, a), 3 mm. ; (spec, b), 3.75 mm. 



Dorsal area bluntly wedge-shape, the summit, ending in the 

 mucro, roundly ridged and bearing about 16 longitudinal striae, 

 which become broken at the sides into rows of elliptical or 

 elongate-subquadrate beads. There are about 14 of these bead- 

 like striae on each side of the dorsal slope, over which they are 

 disposed in a radiately curved manner, and focussed on or aroiuid 

 the mucro. There are two beaded striae to one intermediate 

 and shorter. Area behind mucro, plane, undulate or slightly 

 concave, ornamented with numerous small pustules arranged in a 

 rather obscurely quincuncial pattern. The outer borders of these 

 pustules each carry a pigmented centre, slightly depressed, show- 

 ing the presence of the rudimentary eyes. The articulamentuuj 

 is of a pale creamy yellow, contrasting Avith the pale sage-green 

 colour of the tegmentum. As seen from the under side, it is 

 thickened and wrinkled by divergent ridges on either side of the 

 mucronal pit, and is delicately crenulated on the posterior bor- 

 der. The sutural laminae are produced 1.25 mm. beyond the 

 tegmentum, as seen from the upper surface. 



AfHnities. — Of living species of the sub-genus there appears 

 to be only one well-authenticated example-— viz., Ischnochiton 

 (Ischnoplax) pectinatus, Sowerby sp.,J- whose habitat is in the 

 West Indies (Cuba, (ruadalouite and Barbados). The salient 

 differences between the recent and the fossil form are the den- 



1 Chiton pectinatus, .Sow.; Ann. Mag. Nat. Hist., 1840 (June), p. 288, pi. xvi., fijf. 3. 

 .See also Tryon luxl I'ilsbry, Manual of Conchology, vol. xiv., p. 64, pi. xvii., fiy;s. 25-30. 



