184 MOLLUSCA FROM THE CRAG. 



by their sculpture, as well as by their 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 sustentacula, and the centre or umbo is situated at the 

 posterior margin of that valve ; whereas, in the final valve, or that which covers the 

 posterior extremity of the animal, the projecting portion or umbo is sometimes in 

 the centre ; and the anterior edge has a projecting 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. Bach 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 apophyses, 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 hemispherical 

 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 

 4 Quarterly Journal of the Geol. Society,' Feb. 1846, has formed another division for 

 some secondary fossil and elongated Chitons, under the name of Helminthochiton, 

 or wormshaped 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 inter- 

 mediate periods. 



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

 and the branchiee 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. de Blainville published a monograph of this family in the ' Diet, des Sci. Nat.' 

 xxxvi, with anew arrangement, some of the characters of separation depending upon 



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



