44 U.S. NATIONAL MUSEUM BULLETIN 2 60 



Acidostoma Liljeborg 

 Acidostonta obesum ortum, new subspecies 



Figures 16-18 



Diagnosis of female and juvenile. — Lateral cephalic lobe sub- 

 quadrate, poorly projecting in juvenile, more strongly subconical in 

 adult ; eyes absent ; mouthparts generally like those figured by Shoe- 

 maker (1930) for Acidostoma laticorne Sars except where noted or 

 figured ; mandibular molar a simpler, more slender cone than that of 

 A. laticorne', lower lip similar; inner lobe of maxilla 1 similar in shape 

 but asetose in juvenile ; maxilla 2 weakly to strongly setose ; outer plate 

 of maxilliped similar, lacking medial excavation in juvenile, with small 

 medial folding notch in adult, palp article 4 shorter than in A. lati- 

 corne'^ palm of gnathopod 1 very slightly convex, article 6 stouter 

 than in typical subspecies, armed with evenly spaced, short setae, 

 palm about equal to posterior margin of article 6 in juvenile, slightly 

 shorter in adult ; dactyl of gnathopod 2 straight, formed of two parts 

 (more clearly apparent in juvenile), probably basal portion being 

 armed with partially fused short, thick spine; pereopods 3-5 with 

 sharper distal corners of articles 4-6 than in typical subspecies ; pereo- 

 pod 5 lacking strong spines on anterior margin of article 6; third 

 pleonal epimeron with slightly prolonged posteroventral corner but no 

 distinct tooth ; dorsal margin of pleonite 4 straight ; peduncle of uro- 

 pod 2 expanded but not pectinate or castellate; telson slightly longer 

 than broad, cleft slightly more than two-thirds of its length, similar to 

 A. nodiferum Stephensen (1923). Male unknown. 



Holottpe. — AHF No. 6123, female, 7.5 mm. 



Type-locality.— Station 7231, 27°24'00" N, 115°12'15'' W, 2398- 

 2475 m, Jan. 1, 1961. 



Material. — Holotype and juvenile, 2.8 mm, from the type-locality. 



Remarks. — The head of the adult protrudes strongly below the in- 

 sertion of antenna 2, a situation opposite to that indicated by Sars 

 (1895) in the description and figures of the typical subspecies, A. 

 ohesum ohesum (Bate). The absence of eyes and the slight diiler- 

 ence in shape of gnathopod 1 and pereopods 3-5 are of little systematic 

 importance. Other species in the genus differ from A. ohesum in much 

 more remarkable ways than do these specimens. 



Dahl (1964) calls attention to the taxonomic importance of the pre- 

 formed folding line marked with a medial excavation on the outer 

 plate of the maxilliped. The outer plate is folded around the other 

 mouthparts perhaps to seal off leakage into the sucking chamber 

 formed by the mandibles and lips. Only three of the known seven 

 species of the genus have the medial excavation on the outer plate but 



