from the Silurian of Girvan, Ayrshire. 353 



kind permission, placed two pieces of a very remarkabte fossil in 

 my hands, for the purpose of examination and description. 



The specimens under consideration were obtained by Mrs. Gray 

 from the Upper Bala Beds of Thraive, near Girvan (of Lower 

 Silurian age). 



These pieces, viewed apart, displayed the impression of six some- 

 what oblong imbricated plates, ornamented with lines of growth, 

 coinciding with the lower and anterior borders of each plate ; and 

 also traversed obliquely by a number of transverse radiating furrows 

 or ribs, one of which is more prominent than the rest, running from 

 the posterior dorsal angle of each plate to the anterior lower border, 

 and radiating upwards. When the two pieces are united, a narrow 

 wedge-shaped space remains, once filled with the organism itself, of 

 which the matrix has preserved a faithful copy of the exterior of the 

 two sides. By running in some verj?^ fine plaster of paris, T have 

 been enabled to obtain a cast from this natural mould, which fairly 

 represents in plaster the actual form of the testaceous covering of 

 the lost organism. We at once perceive that each quadrate plate 

 was strongly carinated and was bilaterally symmetrical ; that each 

 plate was imbricated, i.e. it overlapped the succeeding plate, like a 

 row of ridge-tiles upon a house-roof (see PI. IX. Figs. 7, 8, 9). 



The length of the six plates is 35 millimetres, that of the most 

 anterior plate 5 mm. ; of the second 5 mm. ; of the third 6 mm. ; 

 of the 4th 6 mm. ; of the fifth 6 mm. ; and of the sixth 7 mm. ; the 

 average depth appears to be 5 mm. Allowance must however be 

 made owing to the breaking off of the thin margins of the plates in 

 the mould, where one plate has overlapped another. I am inclined 

 to believe that the first plate in the series, which is more rounded 

 off than the rest, was probably the true anterior or cephalic extremity 

 (see PI. IX. Fig. 7a), and that if a part of the organism is lost, 

 which seems equally certain, it was from the other extremity, which 

 is fractured across close to the edge of the sixth plate. 



The first and second plates were apparently rather smaller than 

 the rest ; but the concentric and radiating lines which divide each 

 plate or valve into three parts, a dorsal and two lateral areas, are 

 observable on all. 



What then was the nature of this fossil ? and to what class of 

 organisms should it be referred ? 



So long ago as 1857, Prof. L. G. de Koninck published ' some 

 observations on two new species of Chiton from the Upper Silurian 

 (Wenlock Limestone) of Dudley, namely. Chiton Grayanus, and 

 Ch. Wrightianus. 



The first of these two species, Chiton Grnyanns. has remained un- 

 challenged ; but the determination of the second, Chiton WrighfiannK, 

 was called in question by me in 1865, and in a paper read before the 

 Geological Society in that year, I showed that Ch. Wrightianvs was 

 identical with a recently discovered fossil named by me Turrilepas — 



1 Bulletin de I'Academie Royale des Sciences, etc., de Belgique, 26 Annce, 2nd 

 ser. tome iii. Bruxelles, 1857, pp. 190-199, pi. i. ; see also Ann. and Ma,!?. Nat. 

 Hist. 1860, 3rd ser. vol. vi. pp. 91-98, pi. ii. translation by W. H. Baily, L'\G.S. 



DECADE III. — TOL. II. — MO. VIII. 23 



