178 CEYLON PEARL OYSTER REPORT. 



for the other species of the type ; rostrum slightly shorter than the length of the 

 carapace extending to about the same level as the flagella of the antennules, two 

 ventral spinules at about the level of the eye ; posterior median dorsal spine well 

 developed, arising from the pyramidal base noted above, much shorter than the 

 post ero -lateral spines, a prominent carina running from the rostrum along the middle 

 of the carapace, terminating in the postero-dorsal median spine ; antero-lateral 

 spines rather short, with a prominent spine of equal length arising from their bases 

 and projecting ventrally ; postero-lateral spines well developed, extending backward 

 to the level of the boundary between the second and third segment of the. abdomen, 

 with a secondary spiiiule on each near the base ; no prominent ventro-lateral spine 

 on the carapace midway between the antero- and postero-lateral spines, as seen in 

 A. pyramidalis and A. unidens ; two small spinules on the ventro-lateral margin 

 very near to the point of origin of the postero-lateral spines. 



Abdomen with all its segments well developed, each having their postero-lateral 

 corners very acutely drawn out into spines, rather more so than in A. unidens ; sixth 

 segment with a pair of rather long and acute spines on the median posterior border. 



Telson about as long as broad, six marginal spines well developed and acute ; 

 between the submedian spines there are 32 denticles, that is, 16 on each side of the 

 centre ; between the submedian and intermediate spines on each side there are 

 7 intermediate denticles, and between the intermediate and lateral spines on each 

 side there is a single lateral denticle situated at the base of the lateral spine. 



Appendages of abdomen all well developed, each with a rudiment of the future 

 gills already present. 



Uropods fairly well developed, reaching slightly beyond the lateral spines of the 

 telson ; basal prolongation with the inner spine much longer than the outer, and 

 exhibiting a very slight swelling near the origin of the latter ; external edge of the 

 outer branch showing traces of six spines. 



Raptorial claws (fig. 29) having the dactylus with two developed spines in addition 

 to the terminal one, and showing traces of two more below the integument. 



Length 9 millims. 



The second specimen, which measures 10 millims., agrees perfectly with the above 

 description except that the telson only has 26 denticles between the submedian 

 spines and six denticles between the submedian and intermediate spines on each side. 

 The distinguishing features of this larva are : 



(1) Its small size taken with its advanced state of development ; 



(2) Absence of a prominent ventro-lateral spine on the carapace ; 



(3) The spines arming the dactylus of the raptorial claw ; 



(4) The spinulation of the telson. 



This Alimerichthus at 9 millims. long is at exactly the same stage of development 

 as CLAUS' Alimerichthus at 18 millims. long (GLAUS, 1871, fig. 30). 



