l. r )l 



MORPHOLOGICAL AND DESCRIPTIVE NOTES ON THE GENTS 

 CRYPTOPLAX. 



By Henry A. Pilsrry, Se.D., etc. 



Mead 9th November, 1900. 



(ELATES XIV Axn XV.) 



Tin: genus Cryptoplax is one of considerable interest to the rnorpho- 

 logist, on account of the degeneration of the armour characteristic 

 of the Polypi acophora, and the convergence in outward features to the 

 Aplacophora. If the lack of fossil, or recent, intermediate stages 

 prevents our tracing the descent of the Aplacophora, a parallel process 

 may yet be seen in the Cryptoplacidre. 



The opportunity of describing a new species, very advanced in 

 the degeneration of its valves, has been taken to present certain 

 interesting phases of parallelism between ontogeny and phylogeny 

 among the other species. 



In normal Chitons, the contours of the individual valves have been 

 determined largely by purely mechanical factors. The intermediate 

 valves, in close contact with the ones before and behind, are as 

 necessarily arrested from expansion in those directions as a Crepidula 

 plana growing in a narrow shell. Under such circumstances the 

 Crepidula grows long and narrow, and the Chiton valve grows mainly 

 at the sides. The two terminal valves of the Chiton, on their free 

 sides, expand as roundly and naturally as a limpet. 1 Now in Cryptoplax 

 some of the valves have been freed from the pressure fore and aft, and, 

 as might be expected, they expand freely in length ; but the vermi- 

 form shape of the animal, and perhaps some of the needs of its little 

 known ways of life, prevent the lateral expansion of the valves for 

 which we should otherwise look. 3 



The general structure of the valves and girdle, apart from the 

 features just mentioned, is not very unlike such Acanthochitida? as 

 Notoplax, which, in my opinion, is a comparatively primitive member 

 of the family, and consequently nearer the stock whence the Crypto- 

 placidse arose. 



My studies now show that within the genus there are two widely 

 divergent lines of specialization. In the one, the posterior insertion- 

 plate of valve viii projects backward, as in normal Chitons (PL XIV, 



The posterior sinus in the tail-valve of Mopalia, etc., was mechanically caused by 

 the habit the animal has of raising the median-posterior part of the girdle 

 in order to maintain a free and constant conduit for water from the gills, thus 

 bringing a constant pressure to bear on the growing periphery of the tail-valve 

 in the position of the sinus. 



Arthur Adams, I believe, somewhere writes of breaking coral blocks to get the 

 specimens of Cryptoplax, which enter such narrow crevices of the coral thai they 

 are stretched out to a foot in length. 



VOL. IV. — MARCH, 1901. 11 



