227 



Ischnoradsia papuaensis, n. sp 



General appearance. — Broad, end valves equal in size and as well as the 

 lateral areas radially ribbed, lateral areas raised, a minute granulose pattern 

 covering the whole shell. Girdle scales large, pebbly, outer half keeled and 

 pointed, inner half triangular and imbricating. 



Colour. — Pale or worn valves are Light Greenish-Olive streaked with 

 Mikado-Brown (Ridgway's Colour Standards, pis. xlvi. and xxix.), with often 

 dark dorsal area and several dark spots on the posterior margin ; the dark- 

 coloured valves are green on the dorsal and lateral areas, merging into dark olive- 

 brown towards the posterior margin. 



Inside of shell. — Light greenish-olive merging into darker green, highly 

 polished ; in the median valves the sutural laminae, shallow, mostly bowed 

 inwards in the centre. The tegmentum is infolded from the posterior margin, 

 slits 1-1, and in one valve 2-1 ; the slit ray forms a very deep sinus, with highly- 

 thickened walls, teeth of the insertion plates sharp, slightly grooved on the 

 inside, eaves solid, articulamentum protrudes beyond the tegmentum ; anterior 

 valve 13 slits, posterior 18, sutural laminae with broad sinus similar to the 

 median valves. 



Anterior valve. — This valve is uniformly very minutely decussate; while 

 in juvenile shells little other sculpture is to be seen. At an early stage they 

 develop a series of flat, shallow, radial ribs, which are very broken, often con- 

 tinuing for some distance and then dying away, others being intercalated and 

 taking their place ; this gives the valve the appearance of being covered with 

 dashes. 



Posterior valve. — Broad and flat, mucro ill-defined, anterior of centre. 

 The anterior margin of the tegmentum is bowed forward, the portion of valve 

 in front of the mucro is very short longitudinally, and is decussated in a similar 

 manner to the pleural areas of the median valves. The portion behind the mucro 

 occupies nearly three-quarters of the valve, and is minutely decussate through- 

 out ; but, in addition, half-way down, the valve commences a series of broken, 

 intercalated, radial ribs, similar in character to the anterior valve. 



Median valves. — In the dorsal area the apex is smooth, then becoming 

 grooved, with very minute, somewhat irregular, longitudinal grooving, this again 

 changing, as the anterior margin is approached, into a decussated pattern, com- 

 posed of evenly-distributed, minute granules. In the pleural area the upper 

 part is longitudinally striate, similar to the dorsal, but rapidly becoming granulose. 

 The lateral area is raised, with broad, shallow, radial ribs. On the valve before 

 me I count 8, the sulci between are much narrower than the ribs. The whole 

 area is decorated with minute granules like the rest of the shell. It will be seen 

 that juvenile shells will show no radial ribbing at all, that feature being a later 

 development. 



Girdle. — The girdle is indistinctly banded with brown and light bands. The 

 scales on the lighter portions are dark on the lower or outer half and light only 

 on the upper, nearest the shell. The scales are quite characteristic of this genus, 

 being large, subcarinated or keeled on the outer half, giving the appearance of 

 being pointed, the upper half of each scale is subtriangular, flattened, and imbri- 

 cating; the surface of the scale is not strictly smooth. 



Measurements. — A somewhat damaged specimen measures 20X13 mm. The 

 tegmentum only, of valves of type: anterior, 5X10; posterior, 5^X10; median, 

 3^X121 mm. 



Habitat. — Normanby Island, Papua. 



Remarks. — I have placed this species under the genus Ischnoradsia because 

 its scales are quite characteristic of members of that genus. But one was sur- 

 prised to find only one slit in most of the median insertion plates, one valve only, 



