1921.] B. Prashad : Notes on LamcUibranchs. 143 



deeper ridges ; dorsal margin short, regularly curved, convex ; 

 anterior margin fairly large but shorter than the posterior, con- 

 cave and rather sinuous in the proximal or upper half, then 

 regularly curved to the broad podium, from the latter the ventral 

 margin sharply curves downwards and backwards until in line 

 with the umbo it forms a broad arc and then rapidly curves up 

 again to the gonium ; posterior margin biangulate, regularly and 

 convexly curved above in continuation of the dorsal margin but 

 nearly straight below the upper obtuse angle ; umbones promi- 

 nent but rather small , curved inwards and forwards and separated 

 from one another in the middle line ; ligament long and thick 

 but not greatly projecting ; lunule broad, but not very deep, 

 carinate in the middle. Hinge. — Right valve with four laterals, 

 two anterior and two posterior ; upper anterior small but thick 

 and knob-like ; lower, seen from above, triangular, thick, pad-like 

 with the anterior and posterior margins of approximately the 

 same length ; upper posterior more elongate but less prominent 

 than the upper anterior, appearing only as a slight callosity ; the 

 lower one elongate, ridge-hke but with a low apex a little further 

 ofi from the centre ; between the two anterior and two posterior 

 laterals there is a deep concavity for the fitting in of the single 

 laterals of the opposite valve, the anterior concavity is much the 

 deeper of the two ; of the three cardinals the anterior one is sharp 

 or with only a very shallow furrow across its free edge; it is in- 

 clined forwards, the middle and posterior are both deeply furrowed, 

 appearing somewhat bifid and both inclined backwards. Left 

 valve with two laterals, anterior one very short, thick and stum- 

 py, somewhat conical ; posterior elongate, triangular with the 

 apex lying further from the middle ; 3 cardinals, anterior forward- 

 ly inclined, middle and posterior backwardly, the first two deeply 

 bifid, posterior single. Pallial line with only a shallow sinus. 

 The young shells differ from the adult in being subtrigonal or even 

 subcircular. with the anterior margin nearly straight in its proxi- 

 mal portion and evenly rounded below without a projecting 

 podium ; the posterior margin not or only indistinctly showing 

 the biangulate nature ; the shells are comparatively thicker in 

 diameter, the umbones a little more prominent and the periostracal 

 ridges comparatively more regular and distinct. The series of 

 shells in the Indian Museum well illustrates the change in shape 

 from the young to the adult form. 



Measurements (in millimetres). 



Length .. 46 I 52 54 63 I 74 88 I 90 . 103 



Height .. 44 48 51 62 , 73 85 1 85 I 87 



Thickness .. 26 27.5 ! 28-4 34 38'2 5c 1 5^ i .53 



Distribution. — According to the earlier authors the species 

 was considered to have a wide range from Ceylon to Java, but 

 it has been established by later workers that the Javanse species 

 is quite distinct, and that C. ceyionica is confined to Ceylon only. 



