on the ChitonidcB. 29 



zona tomentosa obvolata, omnino tecta. Zona (in utroque scuto) fissiira 

 porisque duobus tubulosis lateralibus signata : supra scutum anticuni 

 pori quatuor. Branchiae abbreviatae. 



Ex. Ch. porosus, Burrow. 



Has divisiones sub^jjenericas non omnes egomet vidi, at lubenter recepi. 

 Squamae et sculptura semper oculis armatis examinandae. 



These animals frequent the rocks and stones of the sea-coast, and are 

 distributed nearly over the whole globe. Many of the species are con- 

 stantly under water, while others ascend above low or even high water- 

 mark, spending the day exposed to the hottest sun, or selecting a rest- 

 ing-place which is only occasionally moistened by the rude and restless 

 surf. In Chitonellus and Cryptoconchus there are certain minute organs 

 on the zone, which bear a strong resemblance to the spiracula of the 

 annulose animals. From their habit of quitting the watery element, like 

 many of the TurhinidcE, I once supposed that the organs for the aeration 

 of the circulating fluid might be of a compound nature, (pulmono-bran- 

 chiati.) It is, however, far more probable (as in the case of some Crus- 

 taceous* genera which I am now investigating,) that this process is capa- 

 ble of a diurnal or a temporary interruption, or that the branchiae, so 

 long as they are kept moist, and shielded from atmospheric influence, 

 may perform their functions, though much more slowly. 



The species are very numerous, but involved in the greatest confusion. 

 As De Blainville has remarked, " Leur separation en petits groupes na- 

 " turels est assez difficile, nous ne doutons cependant pas qu'on y parvi- 

 " enne, si Ton peut reussir a etudier a la fois et completement les ani- 

 " maux et les coquilles." From the great variation in their colouring, 

 and the liability of the older shells to become corroded and decorticated 

 by atmospheric exposure, the action of salt water, or the blows of roll- 

 ing stones, while the spines and other appendages of the zone, are worn 



* In the decapod short-tailed Crustacea which reside at the bottom of the 

 ocean, the foramina which admit the water to the branchiae are very large: in 

 the genera which dwell lon;^ on land they are contracted. Those curious open- 

 ings, seated at the base of the arms, and closed with a moveable opcrculiform 

 ciliated yrrnua, 1 have termed portnlx. 



