36 ACANTHOCHITES-CRYPTOCONCHUS. 



Valves entirely covered by the girdle except a linear area at the 

 ridge of each. Posterior valve having the insertion-plate with several 

 (5-7) slits, anterior valve 5-slit. Girdle leathery, naked, bearing a 

 series (18) of sutural tufts on tubercles, or pores, sometimes sub- 

 obsolete, along the sides of the valves. Gills extending along the 

 posterior half of the foot. 



This subgenus cannot in fairness be dated from the time of its 

 pullication in Burrow's Elements, for in that work it is in no way 

 defined and is disowned as a valid genus. Burrows simply says that 

 Blainville has affixed the names Cryptoconchus porosus and C. larvce- 

 formis to two specimens in the British Museum. The latter belongs 

 of course to Chitonellus. In 1829, Guilding adopts Cryptoconchus 

 as a genus, and gives a generic diagnosis. Blainville himself ignores 

 the name in his publication on Chitons in 1825, believing it a 

 synonym of Chitonellus. 



This group is much more closely allied to Acanthochites than to 

 Amicida ; its valves being exactly the form which would be pro- 

 duced by a little further covering of the side areas in a species like 

 A. (Notoplax) hemphilli. The backward prolongation of the sides 

 into posterior lobes is just as great in that species ; the main differ- 

 ence being that in Notoplax these posterior lobes are not covered by 

 the girdle. The structure of the tail valve is practically the same in 

 Notoplax, Loboplax and Cryptoconchus. 



A. POROSUS Burrow. PI. 3, figs. 57, 58, 59, 60, 61, 62. 



Shell elongated, all but a linear dorsal area of each valve covered 

 by the integument continued upward from the girdle, but in the 

 dried state showing through it the posterior outlines of the valves. 

 Color when dry dark reddish or blackish-brown. 



The outer layer of each intermediate valve is reduced to a narrow 

 dorsal area, shaped like an exclamation point without the dot (') ; 

 upon each side of the apex, the posterior margin of each valve is pro- 

 duced backward in a rounded lobe, showing plainly through the con- 

 tracted outer skin. A more or less developed groove extends to the 

 lateral slits. Head and tail valves with minute circular exposed 

 dots. 



Interior light blue-green. Anterior valve having 5, median 

 valves 1, posterior 5-7 slits. 



Girdle reddish- or blackish-brown in the dried condition, naked, 

 smooth, leathery ; bearing a series of prominent tubercles each with a 



