790 
CLASS XIII. 
Compare for the Anatomy of Chiton, Pout Testac. utr. Sic. 1. Multivalv. 
pp. 1—10, Tab. 111.; Cuvirr Mém. pour servir a ? Hist. nat. et a V Anat. des 
Mollusq. No. 18, pp. 22—28, Pl. 11. figs. 8—14, and A. Ta. MIppENDORFF 
Beschreibung wu. Anatomie neuer Chitonen, Mém. de VAcad. impér. des Se. 
de St. Pétersbowrg, 6e Série, Tom. vi. 1848, pp. 67—215, av. 14 pl. 
The species of this genus are very numerous and difficult to distinguish 
otherwise than by accurate measurements of the comparative length and 
breadth, the greater or lesser inequality &c. of the pieces of shell, and by 
the nature of the margins of the mantle which are not covered by the dor- 
sal shields. In some species these margins of the mantle cover, as though 
they came together by continued growth, the dorsal shields, so that the 
shell is concealed within them. Of these MipDENDORFF forms the sub- 
genus Cryptochiton (Sp. Cryptochiton Stelleri M1IDDEND. 1.1. Taf. 1. figs. 1, 2, 
Chiton amiculatus SowERBy, Conchol. Illustrations, Chitones, fig. 80, 
REEVE Conchologia system. 1. Pl. 132, 133, fig. 80; from which Chiton 
amiculatus PauL., according to MIDDEND., does not differ), The remaining 
species form the sub-genus Phcenochiton MIDDEND., in which the branchiz 
are sometimes situated backwards and the row of pieces of shell is inter- 
rupted by the mantle (Dichachiton MIDDEND., Chitonellus Lam., Sp. Chito- 
nellus levis LaM., Buatny. Malac. Pl. 87, fig. §); or the pieces of shell close 
upon one another and the branchie are placed round about at the sides of the 
mantle (Hamachiton MippEND.) ‘To this last division belong most of the 
species of the genus Chiton of LAMARcK. On account of its size we notice 
Chiton gigas SPENGL., CuEMN. Conchyl. Tab. 96, fig. 819, Encycl. méth., 
Vers. Pl. 161, fig. 3 (figured in the young state by F. Krauss Siidafrik, 
Mollusk. Tab. 11. fig. 3). 
Compare also on the species of the genus Chiton, SPENGLER Skrivter af 
naturh. Selskabet, 1V. 1, 1797, pp. 62—103, Tab. 6, and J. E. Gray Annals 
of Nat. Hist. xx. 1847, p. 131 and foll. 
Patella lh. (exclusive of several species). Shell univalve, 
covering body entirely above, clypeate or retuso-conical. Branchie 
placed under the margin of mantle, lamellose, lateral, disposed in a 
series surrounding the body. Head with two tentacles acuminate, 
bearing the eyes externally at their base. 
Limpets. yl very numerous genus of molluscs. The intestinal canal 
which, as in the preceding genus, is very long and forms many convolutions 
runs with the rectum upwards, so that the vent is placed under the head 
on the right side, whilst in Chiton it lies in the mid line at the posterior 
extremity of the body. The nervous system presents a band lying trans- 
versely upon the cesophagus and swelling on each side into a ganglion, 
from which the nerves for the eyes, the feelers and the neighbouring parts 
arise. Two nervous filaments descend on each side and form a transverse 
ganglion, from which nerves arise for the foot, the viscera and the circular 
muscle that attaches the animal firmly to its shell. Compare CuviER Mém. 
sur les Mollusg. No. 18, pp. 15—19, Pl. 11. figs. 8—19, and the fig. of 
Patella algira DEsH. in Cuv. R. Ani., éd. ill., Moll. Pl. 66. 
