POLYPLACOPHORA AND SCAPHOPODA. 



cially modified in structure, and become converted into minute eyes 

 which are distributed over the exposed surface of the tegmentum. 



The Chitons are inhabitants of the sea, and they are of rare 

 occurrence as fossils, being more abundant in the Palaeozoic than 

 the Mesozoic or Kainozoic deposits. Under the name of Holochiton, 

 Fischer has united the recent genus Leptochiton and a number of 

 Palaeozoic types which have been placed in groups under special 

 names (Ifelminthochiton, Gryphochiton, &c.) The general characters 

 of the " Holochitons " are the possession of an oval or elongated 

 shell, in which the " insertion-plates " are obsolete or more or less 

 developed, their margins being in the latter case entire, and showing 

 neither fissures nor denticulations. Taken as a whole, the Palae- 

 ozoic Chitons are characterised by the elongated form of the shell, 

 the narrowness of the valves (fig. 733), the absence or rudimentary 



condition of the insertion-plates, and the 

 widely separated sutural laminae. In their 

 narrow valves and elongated form the 

 Palaeozoic Chitons resemble the recent 

 genus Cryptoplax ( = Chitonellus), but the 

 shell in the latter is formed wholly by the 

 articulamentum, the valves are more or 

 less separated, and the insertion-plates 

 have fissured margins. As regards their 

 geological distribution, the " Holochi- 

 tons " range from the Ordovician to the 

 Permian rocks, but the majority of the 

 known forms are Carboniferous. Some 

 of the Carboniferous types, however, 

 possibly belong to Cryptoplax (Chitonel- 

 lus). The conical calcareous plates of 

 the Silurian rocks of Gotland, which 

 have been described under the name of 

 Chelodes, are regarded by Lindstrom as referable to the Chitonidce. 

 The plates of Chelodes show remarkable resemblances to those of 

 the Chitons, but they are altogether peculiar in the fact that not 

 only are there no insertion-plates, but the sutural laminae are also 

 wholly unrepresented. 



But few representatives of the Polyplacophora have hitherto been 

 found in the Secondary and Tertiary rocks, and these few appear to 

 belong to the genus Chiton itself, in which the insertion-plates are 

 well developed and possess fissured margins, those of the cephalic 

 and anal valves being alike in form. 



Fig. 733- — Holochiton 

 chiton) ftriscus, from the Carbon 

 iferous Limestone of Belgium. «, 

 Four consecutive plates ; b, Anal 

 plate seen from the exterior and 

 from the interior. (After De 

 Koninck— copied from Zittel.) 



