96 THE VOYAGE OF H.M.S. CHALLENGER. 



which, starting with Alima gracilis, with its short carapace and elongated hind body, 

 leads, through many intermediate ty^jes, to a larva which is short and wide, and 

 almost completely covered by the elongated carapace, like an Erichthus, although its 

 general structure is Alima-like. For this larva, which he justly regards as the ancestral 

 form from which the Alima larvfe have been produced, Claus proposes the provisional 

 generic name Alimerichthus, as expressing its double relationship, to Alima on the one 

 hand, and Erichthus on the other. 



He figures' a single advanced larva of this type 18 mm. long from the Indian Ocean, 

 but as he gives no account of its early stages, the occurrence in the Challenger collection 

 of younger specimens of this type is a matter of great interest, as these younger larvse 

 show that the young Alimerichthus, like the young Alima, passes through a stage in 

 which the last six thoracic somites have no appendages, while the raptorial limbs of the 

 second thoracic somite and the first pairs of abdominal appendages are well developed 

 and essentially like those of the adult. 



I have selected from a collection made, February 29, 1886.. in the South Atlantic, oft' 

 the coast of South America, in lat. 36° 9' 8" S., long. 48° 22' W., the two specimens 

 which are shown in PL VIII. fig. 8, and PL IX. fig. 3, and which represent a form 

 which is very closely related to, but probably not identical with the one figured by 

 Claus. The youngest, shown in PL VIII. fig. 8, is 8715 mm. long, and the next stage, 

 shown in PL IX. fig. 3, is 15'52 mm. long, while Claus's Alimerichthus is 18 mm. 

 long, so that we probably have, in this series, three successive moults in the history 

 of the larva for which Claus's generic name may be retained without a specific 

 name. Alimerichthus is characterised as follows — A short wide Alima with a short 

 hind body which is wide and flat in the older larvse. The mouth is near the middle of 

 the carapace, and the rostrum is less than half as long as the carapace, which has a 

 median dorsal spine, and moderately long antero- and postero-laterals. There is a second- 

 ary spine on the inner edge of the postero-lateral close to its base, and a very prominent 

 acute spine projecting outwards from the lateral edge of the carapace, about midway 

 between the bases of the antero- and postero-lateral spines, and two or three smaller ones 

 projecting inwards between this and the one at the base of the postero-lateral. The 

 length of the carapace, measured on the middle line, from the tip of the rostrum, makes 

 much more than half the total length, and it covei's all of the thorax except the tip of 

 the eighth somite, while the tips of the postero-lateral spines are in the line of the 

 anterior edge of the telson. The eye stalks are about as long as the eyes, which have 

 swollen globular tips, the width of the carapace equals about one-third the total length, 

 the telson is wider than long, with six marginal spines, and numerous secondary spines 

 between the submedians and also between each submedian and the adjacent intermediate. 

 The inner spine of the basal prolongation of the uropod is slightly longer than the outer, 



I Metamorphose der Squilliden, p. 147, Taf. viii. fig. 30. 



