s 



Shhlii's III! iiiariiK' Oslrncuils 301 



this process is iinii'orinl\- ami weakly roiivex, forming a uniform continuation of tlie ventral 

 margin of the shell; its dorsal margin is, on the other hand, rather strongly concave. The rostrum 

 has a rather prominent, broadly rounded corner anteriorly; this corner is, however, less striking 

 because the ventral portion of tlic antcrioi' margin of th(> rostrum, seen from the side, is covered 

 hv a low. l)i()a(ll\ rounded verruciform process, issuing ventrally on the rostra of both valves 

 and directed forwards and downwards; the ventral corner of the rostrum is almost rectangular. 

 Seen from below (fig. 4) the shell has its greatest breadth at about the middle; the 

 posterior siplion-shaped process is well defined. Seen fro m t li e I i' o n t the shell appears 

 almost circular. The surface of the shell is without ridges and other sculptural 

 elevations except for the above-mentioned low, broadly rounded, verruciform process situated 

 ventrally on the rostrum and a low rounded elevation situated just ventrally of the rostral 

 incisur, a short distance inside the margin of the shell. On the tj-pe specimen this last-mentioned 

 process was somewhat larger on the right valve than on the left one and covered the margin 

 of the shell to some extent when the shell was seen from the side; to judge from the larva (male) 

 that was also investigated, this character seems, however, to vary somewhat. The whole surface 

 of the shell is covered by numerous rounded cavities, which are situated rather close together, 

 are moderately deep and vary to some extent in size, those situated near the margin of the 

 hell being rather small and most of those situated farther in somewhat larger. In addition it 

 is to be noted that on the rostrum, on the part just behind the rostral incisur and on the siphon 

 the surface of the shell, near the margin, has diminutive warts. The pores of the surface are 

 small and moderate in number, either with or without a short, fine bristle. Seen f r o m 

 inside (fig. 5): Medial bristles: On the rostrum there is a sparse row of rather long bristles 

 which runs obliquely forwards and upwards; most of these bristles are bifurcated and are 

 furnished with long, fine hairs arranged close together in the shape of a feather; the others 

 are generally simple and smooth. In addition on the rostrum behind and above this 

 row there is a moderate number of scattered, smooth, short, simple bristles. On the anterior 

 side of the incisur there are a few rather long or rather short bristles, most of them apparently 

 simple and smooth. The two bristles situated close to each othei- near the inner edge of the 

 incisur are bifurcated, smooth or almost smooth, the anterior one being rather long; above these 

 there is a single, bifurcated, rather short, bare bristle. ( )n the list behind the rostral incisur 

 there is a rather dense row of bristles, all rather long, bifurcated and furnished with long 

 hairs situated close together and arranged in the shape of feathers (one of the anterior of 

 these bristles is reproduced in fig. 6). This row of bristles stops suddenly at about a third of the 

 distance along the shell ; a few single, simple, smooth , rather short bristles may, however, be observed 

 on the list along the posterior part of the ventral side of the shell. A few, simple, short, smooth, 

 scattered bristles are to be foimd between the list and tlie inner line just t)ehind the rostral incisur; 

 on the part between the list and the ventral margin of the shell no bristles seem to occur. 

 The inner walls of the posterior process of the shell, the siphon (fig. LII), are almost perfectly 

 smooth, with only weakly developed, reticulate sculpture (giving an apparent effect of scahness) 

 and have a few scattered short simple bristles. Note also in fig. 1 tlu; complicated structure 

 of the joining liiu' and the niai-ginal ])(ires. The exit outwards of the .siphon is a rather wide, 



