790 CLASS XIII. 



Compare for the Anatomy of Chiton, POLI Testae, utr. Sic. I. Multivalv. 

 pp. i 10, Tab. in.; CUVIER Mem. pour servir a VHist. nat. et a I'Anat. dcs 

 Mollusq. No. 1 8, pp. 22 28, PI. in. figs. 8 14, and A. TH. MIDDENDORFF 

 Beschreibung u. Anatomie neuer Chitonen, Mem. de VAcad. imper. des Sc. 

 de St. Petersbourg, 6e Serie, Tom. VI. 1848, pp. 67 215, av. 14 pi. 



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 MIDDENDORFF forms the sub- 

 genus Cryptochiton (Sp. Cryptochiton Stelleri MIDDEND. 1. 1. Taf. i. figs, i, 2, 

 Chiton amiculatus SOWERBY, Conchol. Illustrations, Chitones, fig. 80, 

 REEVE Conchologia system, n. PI. 132, 133, fig. 80; from which Chiton 

 amiculatus PALL., according to MIDDEND., does not differ). The remaining 

 species form the sub-genus Phcenochiton MIDDEND., in which the branchiae 

 are sometimes situated backwards and the row of pieces of shell is inter- 

 rupted by the mantle (Dichachiton MIDDEND., Chitonellus LAM., Sp. Chito- 

 nellus Icevis LAM., BLAINV. Malac. PI. 87, fig. 5) ; or the pieces of shell close 

 upon one another and the branchiae are placed round about at the sides of the 

 mantle (Hamachiton MIDDEND.) 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., CHEMN. Conchyl. Tab. 96, fig. 819, Encycl. meth., 

 Vers. PI. 161, fig. 3 (figured in the young state by F. KRAUSS Siidafrik. 

 Mollusk. Tab. in. fig. 3). 



Compare also on the species of the genus Chiton, SPENGLER Skrivter of 

 naturh. SelsJcabet, iv. i, 1797, pp. 62 103, Tab. 6, and J. E. GR AT A nnals 

 of Nat. Hist. xx. 1847, p. 131 and foil. 



Patella L. (exclusive of several species). Shell univalve, 

 covering body entirely above, clypeate or retuso-conical. Branchiae 

 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. A 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 oesophagus 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 Mem. 

 sur les Mollusq. No. 18, pp. 15 19, PI. n. figs. 8 19, and the fig. of 

 Patella algira DESK, in Cuv. R. Ani., ed. ill., Moll. PL 66. 



