[From the American Journal of Science and Arts, Vol. VI, July, 1873.] 



Tlie Megalops stage of Ocypoda ; by S. I. Smith. — The 

 Mbnolepis inermis, long ago described by Say,* and partially 

 figured by Dana,f is undoubtedly a stage in the development of 

 Ocypoda arenaria. The large size and peculiar structure of this 

 megalops render it one of the most interesting forms of the group 

 of larvae to which it belongs. It is closely allied to the Monolepis 

 orientalls Dana, from the Sooloo Sea, figured in detail on plate 31 

 of the Crustacea of the Wilkes' Exploring Expedition. The cara- 

 pax is very convex above and narrowed toward the front. The 

 front is defiexed and the extremity tricuspidate, the median tooth 

 being long and narrowly triangular, while the lateral teeth are 

 small and obtuse. The sides are high and impressed so as to re- 

 ceive the three anterior j)airs of ambulatory legs. The third pair 

 of ambulatory legs ai-e closely appressed along the upper edge of 

 the carapax, and extend forward over the eyes, the dactyli being 

 curved down over the eyes and along each side of the front. The 

 posterior legs are small and weak, and each is folded up and lies 

 in a groove on the latero-posterior surface of the carapax. The 

 external maxillipeds have almost exactly the same structure as in 

 the adult Ocypoda, and, as in the adult Ocypjoda, there is a tuft 

 of peculiar hairs between the bases of the second and third ambu- 

 latory legs. This megalops is common upon the coast of the 

 Southern States, it has been found at Block Island, and I have 

 myself collected it, late in August, at Fire Island Beach, Long 

 Island. In the largest specimen from the last locality the carapax 

 is 9'4 mm long and 5*6 broad. 



A large number of young specimens of the Ocypoda, collected 

 at Fire Island Beach, indicate plainly that they had only recently 

 changed from this megalops. Some of the smallest of these speci- 

 mens, in which the carapax is 5*6 to 6'0 mm long and 6*1 to 6'5 

 broad, differ from the adult so much that they might very easily 

 be mistaken for a different species. The carapax is very slightly 

 broader than long and very convex above. The front is broad, 

 not narrowed between the bases of the ocular peduncles, and tri- 

 angular at the extremity. The margin of the orbit is not trans- 

 verse but inclines obliquely backward. The ambulatory legs are 

 nearly naked, and those of the posterior pair are proportionally 

 much smaller than in the adult. 



The adult Ocypoda is terrestrial in its habits, living in deep 

 holes above high water mark on sandy beaches ; but the young 

 in the zoea state are undoubtedly deposited in the water, where 

 they lead a free-swimming existence like trae pelagic animals, 



* Journal Acad. Nat. Sci. Philadelphia, vol. i, p. 15V, 1811. 

 f Crustacea Wilkes' Bxpl. Exped., plate 31, fig. 6. 



