210 CHITONID^E. 



geneous texture as the external." The tubular structure 

 of the outer layer appears to be accompanied by the ab- 

 sence of an epidermis, respecting which I offered an 

 explanation in my account of the Brachiopoda at p. 6 

 of Vol. II. The second volume of the ' Zoological Jour- 

 nal' (1825) contains an accurate description, by the 

 Rev. R. T. Lowe, of some Scotch Chitons ; and Baron 

 Middendorff has given an elaborate essay on the Russian 

 kinds, with details of their anatomy. The genus abounds 

 in species, which are all more or less gregarious. Reeve 

 has lately enumerated 189, and this list is not complete. 

 The British Chitons live attached to rocks, stones, and 

 old shells j they inhabit various depths of water, and 

 many live between tide-marks. Some of their shelly 

 plates occur in upper tertiary strata ; others of extinct 

 form have been found in older and even ancient forma- 

 tions. Gray has made twenty genera out of the one so 

 familiar to us by name. I do not consider it necessary 

 to apply this rate of multiplication to our native species : 

 the following conspectus may suffice to distinguish 

 them : 



A. Girdle covered with spines, and having also tufts of bristles. 



(Acanthochites, Leach, fide Risso.) 1. C. fascicularis. 



2. C. discrepans. 



B. Girdle spinous, without tufts. (Acanihopleura, Guilding). 



3. C. Hanleyi. 



C. Girdle covered with scales or granules. (Lepidopleurus, 



Leach, fide Risso.) 4. C. cancellatus. 5. C. albus. 

 6. C. cinereus. 7. C. marginatus. 8. C. ruber. 



D. Girdle apparently reticulated. 9. C. Icevis. 



E. Girdle membranous. 10. C. marmoreus. 



