450 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxv. 



Subfamily FJ^NTDJ^ttlNJE. 



Genus ACHTHEINUS, new. 



Diagnosis. — Female. First thorax segment joined with the head 

 to form the carapace; second and third thorax segments fused and 

 furnished with a single pair of large rounded dorsal plates. Fourth 

 .segment free and armed with a similar pair of plates. Genital seg- 

 ment much smaller than the carapace, its fused dorsal plate with a 

 large median posterior sinus, enlarged at its base similar to that in 

 Perissopus. 



Abdomen small and wholly concealed beneath the genital segment, 

 except what is visible through the posterior sinus of the latter. Anal 

 lamina 1 large, visible beyond the border of the genital segment. 

 Frontal plates completely fused with the carapace; second antennae 

 stout and armed with a powerful claw. Mouth-tube long and slen- 

 der; first maxilla? lacking; second pair close to the base of the mouth- 

 tube, short and tipped with a minute spine. Second maxillipeds 

 with a swollen basal joint and a slender terminal claw. All the 

 swimming legs biramose, rami of first three pairs two-jointed, of 

 fourth pair one-jointed. All the rami rudimentary and armed with 

 spines only; each exopod reinforced on the outside by a well-defined 

 papilla, tipped with a stout spine. 



Type-species. — . I <h thein us oblongus. 



(Achtheinus, axOsivog^ annoying or troublesome.) 



This new genus stands between Dana's Pholidopus (Lepidopus) 

 and Steenstrup and Liiken's Perissopus and is closely related to both. 

 It is like Pholidopus in possessing but two pairs of dorsal plates on 

 the thoracic segments. It resembles the male of Perissopus in the 

 structure of its swimming legs and some of the mouth parts, but it 

 also differs materially from both these genera. 



Pholidopus has the terminal joint of the second maxillipeds flat- 

 tened into a broad lamina covered with scales; the first swimming 

 legs are uniramose and three-jointed; second, third, and fourth pairs 

 biramose, rami of third and fourth pairs one-jointed and rudi- 

 mentary. Perissopus has three pairs of dorsal plates on the thorax 

 segments; terminal joint of second maxillipeds enlarged into a kid- 

 ney-shaped adhesion pad, destitute of pinchers, knobs, or claws; legs 

 all biramose, rami of third and fourth pairs one-jointed, minute, and 

 rudimentary. These radical differences make it necessary to establish 

 a new genus for the present species. 



ACHTHEINUS OBLONGUS, new species. 



Plate LXXIII. 



Host and record of specimens. — Two adults and one chalimus fe- 

 male were obtained from a leopard or cat shark, Triakis semifascia- 



