No. 1633. SOME NEW ATLANTIC I SO POD A— RICHARDSON. 



81 



This species differs from the other species of the genus, found on 

 the Atlantic coast, M. fabricii Kr0yer and Munna kr0yeri Goodsir, 

 in the absence of eyes, in having, instead of peduncle-like projections 

 of the head, spine-like processes, and in the truncate character of the 

 terminal segment of the body. 



Family DESMOSOMID^. 

 ISCHNOMESUS, new name. 

 The above name is suggested for the genus Ischnosoma Sars, which 

 is preoccupied, having been used for a genus of fishes in 1829, and 

 also in 1832 for Coleoptera. Sars used the name for Crustacea in 

 1866, the type of the genus being Ischnosoma hispinosum. In the 

 typical form the uropoda consist of two articles, and are bi-articulate. 

 Later Sars describes another species which he also refers to the genus 

 Ischnosoma, I. quadrispinosum. In this form the uropoda consist 

 of a single article. I would suggest for this form the generic name 

 Haplomesus. The two forms described by Beddard as Ischnosoma 

 bacillus and Ischnosoma bacilloides have the uropoda bi-articulate as 

 in the type-species, but the abdomen is composed of Wo segments in- 

 stead of one as in that species. For these two forms I would suggest 

 the generic name Rhabdomesus. For the forms described by Beddard 

 as Ischnosoma thomsoni and Ischnosoma spinosum and the form 

 described by Tattersall as Ischnosoma greeni, all of which agree in 

 having the uropoda formed of a single article and the abdomen com- 

 posed of two segments, I would suggest the generic name Heteromesus. 

 The following key gives the generic characters in a concise form : a 



a. Uropoda consisting of two articles; third article of peduncle of second 



antennae not elongate. 



6. Abdomen consisting of a single segment; fourth and fifth segments of 



thorax elongate, not produced in spmes-Ischnomesus, I. bispinosus (Sars). 



Abdomen consisting of two segments ; fourth and fifth segments of thorax 



exceedingly narrow and elongate, and produced laterally in spines, 



[r. bacillus (Beddard). 

 Rhabdomcsu-s b { 7 -77.7 /T > ^ 

 |r. bacilloides (Beddard). 



a'. Uropoda consisting of a single article; third article of peduncle of second 

 antenna? elongate. 



b. Abdomen composed of a single segment; peduncle of first antenna? com- 

 posed of three articles Haplomesus, h. quadrispinosus (Sars). 



b f . Abdomen consisting of two segments; peduncle of first antenna? composed 

 of two articles, 



h. thomsoni (Beddard). 

 h. spinosus (Beddard). 

 Heteromesus { h. greeni ( Tattersall ) . 



h. spinescens, new species. 

 h. granulatus, new species. 



a A similar arrangement has been suggested by Tattersall, Fisheries, Ireland, 

 Sci. Invest., 1904, II [1905], p. 22. 



& The antenna? in these two species were lost, but I have placed them in this 

 section because of the character of the uropoda. 

 Proc. N. M. vol. xxxv — 08 6 



