ISOPODA. 3 



Tanais gracilis, Heller Plate I. (D). 



1866, Tanais gracilis, Heller, ' Novara Exp., Zool.,' vol. 2, pt. 3 (Crustacea), p. 133, pi. 12, fig. 3. 



Heller's description is as follows : " The head very short, the roundish black 

 eyes placed forward on the somewhat projecting lateral angles, the front bluntly 

 triangular. The lower antenna? five-jointed, thinner than, but almost as long as, the 

 upper, which are three-jointed. The first [coalesced] pera?on segment is the largest of 

 all, narrowed forward, the second [first free] segment the shortest, the fifth, sixth and 

 seventh sub-equal one to the other. The pleon narrows gradually backwards. The 

 first three pleon segments shorter than those of the perreon, but longer than the 

 fourth and fifth pleon segment, the last (sixth) again larger and apically rounded. 

 The uropods (Schwanzanhiinge) five-jointed, their basal joint thick, triangular, the 

 first joint of the appendage (des Anhanges) tolerably long, towards the end somewhat 

 thickened, the four following somewhat shorter than the first, among themselves 

 subequal, cylindrical, the last blunt-ended, all the joints beset with long seta?. The 

 first chelipeds large and strong. The legs little setose. The colour of the body on 

 the surface yellowish or brownish, with dark points and markings. Locality : 

 St. Paul." Length, 3 millims. 



In Heller's account there are one or two ambiguities, for, while he speaks of and 

 figures six segments in the pleon, he uses the expression " fourth and fifth pleon 

 segment " as if intending to mention a composite segment, and, while he distinctly 

 says that the uropods are five -jointed, he appears to distinguish a peduncular joint 

 from a five-jointed ramus. His figure shows only five joints in all, as in our 

 specimen, which was not otherwise separable from Heller's. The mouth organs 

 agree with those in the genus Tanais. The upper antenna? have a fourth joint in the 

 shape of a tubercle, representing the flagellum. The finger in the hinder group of 

 pera?opods is strongly uncinate. The pleopods are delicate in structure. Whether 

 there were more than three pairs I did not succeed in ascertaining. 



Length of specimen, 2 millims. 



Locality : Gulf of Manaar. 



In T. brasiliensis and T. normani the uropods are six-jointed, in T, willemoesii 

 nine-jointed, in T. hirsutus about twelve-jointed, in T. robustus four-jointed. 



Heterotanais, G. O. Sars. 



1880, Heterotanais, Sars, 'Arch. Naturv. Kristian.,' vol. 7 (1881), separat, p. 28. 



1886, Heterotanais, Saks, 'Arch. Naturv. Kristian.,' vol. 11, p. 333. 



1880, Heterotanais, Norman and Stebeixg, 'Trans. Zool. Soc. London,' vol. 12, pt. 4, p. 108. 



1896, Heterotanais, Sars, ' Crustacea of Norway,' vol. 2, pt. 1, p. 13. 



1897, Heterotanais, Dollfds, ' M^m. Soc. Zool. France,' vol. 11, p. 38. 

 1901, Heterotanais, H. Richardson, 'Proc. U.S. Mus.,' vol. 23, p. 501. 



The leading feature in this genus is the sexual difference in the first gnathopods, 

 which are small and normally chelate in the female, but large and complexly 



B 2 



