REPORT ON THE CRUSTACEA MAORTJRA. :\s:\ 



condition of the adult form, while the fifth and sixth pairs are in an early stage of 

 gemmation. 



The pleopoda are represented by Suhm as being single-branched, whereas in my 

 specimen they are all, excepting the first pair, more or less perfectly biramose. 



The sixth pair differs only in being more slender, with the margins parallel, and in 

 having the tooth on the outer margin nearer the basal articulation. The appendage 

 is long and narrow when compared with that which I have given. The telson is short, 

 as is relatively shown in the figure (at /). Suhm remarks that it much resembles 

 the typical features of a young Leucifer. 



Observations. — By tracing the several stages, we may safely conclude from the 

 direct structural affinities, that Mastigopus is a young Sergestes, and that it is preceded 

 by the Acanthosoma stage. But the interval between Acanthosoma and Elaphocaris is 

 so great that a comparison of all the various forms which we believe to belong to 

 distinct species requires us to insist upon a clearer demonstration of the developmental 

 connection between the two. If w r e could bridge over this chasm we might safely trace 

 the sequence, if not in a single direct line, certainly through closely allied genera, to the 

 Elaphocaris, shown by v. "Willemoes Suhm to have left the ovum so recently that the 

 yolk-mass had not been all absorbed when he observed it, 



Sergestes intermedins, n. sp. 



Rostrum one-third the length of the carapace, armed with a tooth on the dorsal crest, 

 just above the orbital margin, and with a small denticle just within the posterior margin 

 in the dorso-median line ; the rest of the carapace is smooth, excepting a small tooth on 

 the hepatic region above the point of the mandibular attachment, 



Pleon having the five anterior somites subequal in length ; the sixth about twice that 

 of the fifth somite. The anterior three are dorsally smooth, the posterior three are 

 armed with a small tooth at the posterior dorsal margin, the last of which is horizontally 

 produced ; all the somites are furnished with a small tooth projecting from the lateral 

 walls, and the ventral surface is armed in the median line with a large longitudinally 

 compressed spine-like tooth between each of the anterior five pairs of pleopoda. 



The telson is about half the length of the sixth somite and terminates in two small 

 points. 



The ophthalmopoda are nearly twice as long as the rostrum, and the ophthalmus is 

 obliquely mounted on, and broader than the stalk, and fungiform in appearance. 



The peduncle of the first pair of antennae is enlarged at the base for the reception of 

 the otocyst, reaches beyond the ophthalmus, and terminates in two tiagella, of which the 

 Longer is broken off, and the smaller is uniarticulate and rudimentary. 



The second pair of antennas is broken off at the extremity of the peduncle; a long and 



