275 



filling up the antennular fossa. A curious and much aherrant genus, /Ia/>a/o}iofus {=^ Afa/acoso?/uz), 

 with its globular and somewhat membranaceous carapace, connects the ^Isthcnognathinae with 

 the Pinnotherinae. It is not at all astonishing, then, that, the limits of the present subfamily 

 are differentl}' drawn, according to personal opinion of authors: Opisfliopiis has been referred 

 by Alcocic to the Pinnotherclitiae, Tritodynamia and ]'ocltzkowia by their original authors to 

 the Hexapodinae, afterwards by Alcock and Nobili to the Pinnothcrelinae, etc. 



In the present paper all the genera of Pinnotheridae, in which the ischium of 

 the extern al maxiUipeds, which are parallel to each other, is at least as long as, 

 and gene rally even distinctly longer than, the merus, are included within the 

 Asthenognathinae. This criterium, artificial as it may appear, is nevertheless the only one to 

 discriminate the subfamily. 



Key to the genera : 



1 . Last pair of legs considerably reduced in size, reaching about 



to end of meropodite of preceding pair 2 



Last pair of legs not very much weaker than preceding pair . 4 



2. Carapace generally roughly hexagonal. External maxillipeds 



widely distant, ischium and merus separated by a distinct 



suture. Infra-orbital ridge present 3 



Carapace with the lateral margins regularly curved. External 

 maxillipeds close together; ischium and merus broad, nearly 

 equal, subquadrate, separated by an indistinct suture ; palp 

 very small, inserted near antero-external angle of merus and 

 consisting of three subequal segments, placed end to end. 

 Fifth abdominal segment of cf constricted. Fingers of chela 

 gaping,' movable finger with a quadrangular tooth at inner 

 margin Mortensenella Rathbun 



3. Merus of external maxillipeds slightly shorter than ischium, seg- 



ments of palp placed end to end Asthenognathus Stimpson 



Merus of external maxillipeds as long as ischium, dactylus of 



palp inserted at inner border of propodite Tritodynamia Ortmann 



4. Carapace globular, vaulted, membranaceous, adorned with a 



reticulating pattern of brown markings Hapalonotus Rathbun 



(= Rlalacosonia de Man) 



Carapace not vaulted transversely, hard, shining or granulate . 5 



5. Carapace nearly circular, but somewhat broader than long, regions 



not defined. External maxillipeds with the palp larger, the 

 dactylus being triangular^ distally truncate and inserted at 

 inner border of propodus, overreaching the latter segment. 

 Walking legs unarmed, dactyli well developed, half as long 

 as preceding propodites. Species of California, living in Holo- 



thurians and in the Mollusc Lucapina criiiulata Opisthopus Rathbun 



127 



