141 



Choriplax but absent in Amicula, namely, the sub-obsolete 

 or lacking slits, in the median valves and the posterior lobes 

 of the articulamentum uniting across the median line, causing 

 the apices of all valves to be removed inwards from the pos- 

 terior edge. 



While Dr. J. Thiele (Rev. des Sys. der Chitonen, pt. ii. r 

 pp. 106 and 116) leaves the genus Choriplax under the 

 Lepidopleuridae between Hanleya, Gray, and Oldroydia , Dall, 

 did so evidently with considerable misgivings. I cannot 

 follow him in placing the two genera Katharina, Gray, and 

 Amicula, Gray, under the Mopaliidae, and think Pilsbry is 

 right in placing them immediately before the genus Crypto- 

 chiton . I would also suggest the recognition of the genus 

 Cryptoconchus, Blain. and Guilding, with C. 2? orosus > Bur- 

 row, as type, and placing it between Loboplax, Pils., and 

 Katharina, Gray. With its striking development, of the 

 articulamentum posteriorly, in two lobes, its reduced teg- 

 mentum, and pores often sub-obsolete, it seems a sort of 

 "half-way house" between those genera. 



Finally. — I propose that the genus Choriplax, Pils., be 

 taken out of its previous setting amongst the less specialized 

 group, the Lepidopleuridae, and be placed under the Family 

 Acanthochitidae, Pils., following the genus A micula, Gray, 

 and preceding the genus Cryptochiton, Midd. and Gray. The 

 apparent absence of slits in the insertion plates is, I suggest, 

 probably due to modifications in a very specialized form, 

 brought about by the peculiar habits of the chiton. The same 

 tendency is already apparent in the genus Cryptochiton where 

 the slits in the median valves have either been lost entirely 

 or become sub-obsolete. 



Revised definition of the Genus Choriplax, Pils. — Valves 

 are almost covered by the extension of the girdle over them,, 

 the tegmentum or exposed part being reduced to a small, 

 heart-shaped portion, exposed at the apex of each; the inser- 

 tion plates highly developed, smooth and thin, extending 

 posteriorly in a deep lobe on each side, the lobes united across 

 the median line, causing the apices of all valves to be removed 

 inward from the posterior edge. The sutural laminae are 

 apparently shallow, united across the median line, and the 

 sinus reduced thereby to a mere inward bend. Girdle thin,, 

 horny, minutely granulose. 



If we are justified in placing the genus Cryptochiton 

 under the Subfamily Crypt ochitoninae, may we not be justified 

 in doing likewise for this remarkable genus, retaining Adams 

 and Angas' name under the name of a subfamily called Micro- 

 plaxinae with Choriplax f/rayi, Ad. and Aug., as the type, 

 taking the foregoing description as the definition of the sub- 

 family, with the addition of any new features the later 



