GENUS DICHELASPIS. 119 



ference between them. Consequently, I at first united 

 Psecilasma and Dichelaspis, but the latter forms so natural 

 a genus, and is so easily distinguished externally, that I 

 have thought it a pity to sacrifice it. The carina, (which 

 seems to afford better characters than the other valves in 

 Dichelaspis,) from generally running up between the terga 

 and in ending downwards, in three of the species, in a 

 deeply notched disc or fork, more resembles that in 

 Lepas than in Paecilasma ; in the manner, however, in 

 which the imbedded disc, in D. Wanvic/m and D. Grapi, 

 nearly cuts off the inside of the capitulum from the 

 peduncle, there is a resemblance to Pcecilasma eburnea. 

 In the extent to which the valves are separated from 

 each other, in the bilobed form of the scuta, (the two 

 segments in Dichelaspis, perhaps, answering to the upper 

 and lateral projections in the scuta of Conchoderma vir- 

 gata^) and in the basal half of the scuta not descending 

 to the base of the capitulum, there is a considerable re- 

 semblance to Conchoderma ; in both genera the adductor 

 muscle is attached under the umbones of the scuta ; but 

 the structure of the mouth and cirri and caudal appendages 

 shows that the affinity is not stronger to Conchoderma than 

 to Lepas. It appears at first probable, that Dichelaspis 

 would present a much closer affinity to Pcscilasmajissa, in 

 which, owing to the scuta being formed of two segments, 

 there are seven valves, than to any other species of 

 that genus ; but in P.fissa the primordial valve is trian- 

 gular and is situated on the basal segment, whereas, in 

 Dichelaspis, it is elliptic and is seated between the two 

 segments, and is more in connection with the occludent 

 than with the basal segment ; and this I cannot but think 

 is an important difference : in other respects, P. fissa 

 shows no more affinity to Dichelaspis than do the other 

 species of the genus. Finally, I may add that Dichelaspis 

 bears nearly the same relation to Psecilasma, as Con- 

 choderma does to Lepas. 



