354 Dr. H. Woochcard — On Relininthoc/iiton — 



liaving as many as four rows of plates in its test ; that it agreed 

 with this newly-discovered fossil in having the two sides of each 

 principal row of plates unsymmetrical ; that it could not be 

 referred to Chiton, because it had more than eight plates in the 

 series ; and that the surfaces of the plates were uniformly sculptured, 

 and not divided into areas, as in Chiton proper.^ 



Specimens of Turrihpas have been obtained from the Silurian 

 rocks of Bohemia, and described by the late M. J. Barrande under 

 the name of Plumulites (Syst. Silur. de la Boheme, vol. i. Suppl, 

 1872, p. 565). 



A species has also been described and figured by E. Etheridge, 

 jun. (Proc. Eoy. Ph. Soc. Edin. 1878, vol. iv. p. 166, tab. 2. figs. 1 and 

 2), from Balcletchie, near Girvan, in a fine-grained greenish Silurian 

 Mudstone. (See Mon. Silur. Fossils, Girvan District, Ayrshire, by 

 H. A. Nicholson and R. Etheridge, jun., vol. i. 1880, pp. 214-215, 

 pi. xiv. figs. 22-27.) 



I have compared Mrs. Gray's specimen from Thraive with 

 Tnrrilepas from the Wenlock Shale, and I find that it differs in the 

 following particulars. 



Mrs. Gray's specimen has the two sides of each valve bilaterally 

 symmetrical. In Turrihpas the two sides of each valve are un- 

 equal. The ornamentation upon the valves of the Thraive fossil is not 

 uniform, being partly composed of concentric lines of growth, and 

 partly of radiating ribs or strife, which divide each valve into three 

 areas (a median and two lateral ones). 



The ornamentation in Turrihpas consists of uniform fine wavy 

 striee, which cover the whole surface of each valve. Finally, Tur- 

 rihpas is known to consist of several rows of imbricated plates, 

 which vary in form, but not in ornamentation ; whereas Mrs. Gray's 

 fossil has its shell composed of a single row of imbricated plates 

 closely resembling each other in form. 



The form of the plates in the Girvan fossil is peculiar, but it most 

 nearly resembles that of Salter's genus Helminthochiton, and it is 

 to this genus I venture to refer it. 



Family Chitonid^. Genus Chiton, L. Shell composed of eight 

 transverse imbricating plates, lodged in a coriaceous mantle, which 

 forms an expanded margin round the body. The first seven plates 

 have posterior apices ; the eighth has its apex nearly in front. The 

 six middle plates are divided by lines of sculpturing into a dorsal 

 and two lateral areas. All are inserted into the mantle of the 

 animal by processes (apophyses) from their front margin. The 

 posterior plate was considered homologous with the limpet-shell by 

 Dr. J. E. Gray ; the other plates appear like portions of its slope, 

 successively detached. The border of the mantle is either bare, 

 or covered with minute hairs or spines (S. P. Woodward). 



Sub-genus Helminthochiton, Salter, 1847. Quart. Journ. Geol. 

 Soc. 1847, vol. iii. pp. 48-52. 



Form elongate, plates subquadrate, thin ; apex of the anal plate 



1 See Quart. Journ. Geol. Soc. 1865, vol. xxi. pi. xiv. figs, la, b, c, d, e, f, g, h, 

 i, h, I, pp. 486-489. 



