184 MOLLUSCA FROM THE CRAG. 



as by tlicir form. For the information of those collectors to whom it is not known 

 I may observe, that the anterior or primal valve has no projecting processes or susten- 

 tacula, and the centre or umbo is situated at the posterior margin of that valve; whereas, 

 in the fiial vale, or that which covers the posterior extremity of the animal, the pro- 

 jecting portion or umbo is sometimes in the centre ; and the anterior edge has a pro- 

 jecting process or sustentaculum* on each side, upon which the adjoining valve 

 rests ; similar processes project on the anterior edge of each succeeding valve, and 

 support the one adjoining up to the primal valve, which covers the head. By the 

 sliding of these valves over each other a considerable movement is given to the animal, 

 which is enabled to roll itself up after the manner of the Oniscus ; the whole length 

 of the valve is only exposed when the back of the animal is at its greatest tension. 

 The form and size of these sustentacula vary in all the species I have examined, and, 

 when well preserved, will materially assist in specific determination. Each of the 

 central valves is diagonally divided into two areas by a line more or less distinct, 

 passing from a projecting central point or umbo at the posterior part to the anterior 

 lateral edge One is called the dorsal and the other the lateral area, and the shell 

 is increased by the addition of calcareous matter deposited at the anterior and lateral 

 edges of each valve from the central posterior point or umbo, and not around them, 

 the external and ornamental markings being superadded to the sustentacula or apo- 

 physes, as they are called by M. Koninck, and which are always buried within the 

 mantle. The primal valve, or that which covers the head, is generally of a hemi- 

 sperical form, and may be considered as an enlarged dorsal area, with radiating 

 markings, while the posterior or final valve has the lateral areas prolonged or extended 

 round the termination behind the umbo. 



This genus was instituted by Linnaeus. Lamarck separated those eight-valved 

 Cyclobranchiate animals in which the valves were placed upon the back at a distance 

 from each other, to which he gave the name Chitonellus ; and Mr. Salter, in the 'Quar- 

 terly Journal of the Geol. Society/ Feb. 1846, has formed another division for some 

 secondary fossil and elongated Chitons, under the name of Helminthochiton, or worm- 

 shaped Chitons, in which the valves are longer than wide, but that character is also 

 found in some recent species. These latter shells are found in the Silurian or 

 Protozoic Rocks, and the genus is probably continued through all the intermediate 

 periods. 



Animals belonging to this order are said to have a double generative system, and 

 the branchiae are situated under the margin of the shell. They are generally found 

 adhering to stones or upon rocky coasts, seldom ranging deeper than twenty-five 

 fathoms, and some are found above low-water mark. 



M. dc Blainville published a monograph of this family in the ' Diet, des Sci. Nat.' 

 xxxvi, with a new arrangement, some of the characters of separation depending upon 



* These projecting processes are called by Mr. Gray plates of insertion. 



