78 



Family Ptenoplacidie. 



Carapace flat, depressed. Front extremely narrow, projecting freely far 

 beyond the interantennulary septum. Orbits imperfect. No distinct antennu- 

 lary fosste. Epistome none (linear) : buccal cavity broad, subquadrate : external 

 maxillipeds subpediform and aifording almost no concealment to the underlying 

 {i. e. overlying) parts, the palp articulating at the summit of the narrow merus. 

 In neither sex does the abdomen nearly cover the sternum between the bases of 

 the fourth pair of perteopods. 



[Fifth pair of perseopods are subdorsal in position, are reduced in size, and 

 arise near the middle line of the body.] Type Ptenoplax notopus. 



At first sight, from its general shape, from its elongate third pair of trunk- 

 legs and its almost rudimentary notopodal fifth pair, from its extremely in- 

 complete orbits, from the absence of antennulary foss«, and from the curiously 

 small and slender external maxillipeds, Homolid aflinities are suggested ; but 

 that this singular form has nothing to do with the IlomoUdse is shown : (1) by 

 the position of the openings of the oviducts, which is typically Cancroid ; (2) by 

 the form and position of the openings of the efferent ducts of the male, which 

 are typically Catometopan ; and (3) by the number and disposition of the bran- 

 chiae, of which there are only six on each side. 



In the number and arrangement of the branchiae, as well as in the position 

 and degenerate form of the fifth pair of legs and the obsolete epistome, it might 

 be supposed that it had affinities with the Dorlppidae [Borippe and Ethusa more 

 especially). That this is not the case is shown (1) by the position, above in- 

 dicated, of the genital openings of the male ; (2) by the great broad buccal orifice, 

 which is only very partially covered by the maxillipeds ; (3) by the form of the 

 carapace, which is broad, and completely covers the thorax ; (4) by the form of 

 the antennules, which are not obliquely or almost vertically folded in distinct 

 fossge as they are in Ethusa and Dorlppe ; and (.5) by the form of the sternal 

 plastron, which in our new form is a broad pentagonal plate as in many Ocy- 

 podoids. 



It is a true Catometope, but of an archaic type. 



Ptenoplax, Alcockand Anderson. 



Archs'Oflas:, Alcock and Anderson, J. A. S. B. Vol. LXtll. pt. 2, 1894. p. ISO. 

 Ptenoplax, Alcock and Anderson, 111. Zool. Investigator, Crust, pi. xv. 181*5. 



Carapace transverse, greatly depressed, with the front very narrow, and 

 declivous, yet forming a distinct rostrum (f e., its front border is not fused witli 

 the epistome, but is free). Abdomen in both sexes narrow, not nearly co-exten- 

 sive in breadth with the sternum between the penultimate jiair of trunk-legs. 



