COOKE: ON cnyPTOPLAX EVANESCENS, N.SP. 321 



especially in the more buried valves ; the angle of the articulameiitum 

 is very deeply cut, especially in valve 3. 



The sculpture of the external portion of the valves is rude, and 

 consists mainly of a central ridge, which slightly projects forward, 

 forming a sort of blunt beak; this ridge in valve 1 is broadly oval, 

 and lies on the hinder portion of the shell ; in valve 2 it is prominent, 

 and runs the whole length of the valve, with two small, longitudinally 

 striated, lateral areas, but in valves 3, 4, and 5 it consists of a beak 

 only ; this beak area increases in size, and becomes more prominent, 

 in the posterior valves. 



Radula normal (Professor H. M. Gwatkin). 

 Lencfth of full-grown specimen, 4^ inches. 

 Hah. — Funafuti, South Central Pacific (Mr, J. S, Gardiner). 

 The nearest ally of this species appears to be Cr. Burrowi, 

 E. A. Smith, from which it differs markedly in (1) its narrower 

 form, (2) the smaller size of the valves, (3) the relative position of 

 the valves, and (4) length of the gill-rows, which in Burrowi are 

 longer in proportion to the whole length of the animal. 



I am not able to say whether pores are present, but there is no 

 sign of lateral tufts on the integument. The usual three slits in 

 the anterior valve are present, but are very rude and undeveloped. 

 Measurements of the valves are subjoined (in tenths of an inch) : — 



Length. Breadth. 



Front valve 



Second ,, 



Third ,, . . . 



Fourth ,, 



Fifth ,, . . . 



Sixth ,, . . . 



Seventh ,, 



Posterior , , 

 The whole animal is thus thirty-six times as lon< 

 valve, and ninety times as long as its shortest. 



This species may be regarded as forming, so far as our present 

 knowledge extends, a sort of last term in the series of Chitons which 

 exhibit gradual degradation of the valves.^ So far as I am aware, 

 it is the only species yet described in which, while all the valves are 

 reduced in size, some are so far embedded in the integument that in 

 fresli specimens they are scarcely visible. 



If we may take it for granted that the original object of a molluscan 

 shell was the protection of some vital part or parts of the organism, 

 it is ])lain that in the present case, as compared with the foim of 

 shell normal in the Polyplacophora, this particular function of the 

 shell has practically ceased to exist. But, since the vital organs may 

 be taken to exist as before, and to be in equal need of some sort of 

 protection — except in so far as it is afforded by increased safety 

 of habitat — it may be safely assumed that the degradation of the 

 valves has been accompanied by a parallel thickening or extension of 

 the dorsal integument, so that the protection once afforded by one 

 portion of the animal has been transferred to another. 



^ See Haddon, Challenger Expedition, Eeport on the Polyplacophora. 



