338 F. R. Cowper Meed — New Fossil from Girvan. 



Surface of all eight plates covered with, closely set pores (with 

 a few larger ones interspersed) arranged concentrically in a somewhat 

 obscure manner between the finer striae near lateral margins. 



Girdle covered with densely felted mass of small spicules. 



Dimensions. • mm . 



Length of first (anterior) plate . . . about 6 to 7 



Length of other plates 

 Width of first plate 

 Width of other plates 

 Estimated length of whole body 



12 ,, 13 



10 ,, 12 

 22 ,, 24 

 85 „ 90 



Horizon and Locality. Starfish Bed (Upper Bala), Drummuck Group : 

 Thraive Glen, Girvan. (Mrs. Gray's Collection.) 



Remarks. There are several more or less perfect specimens of this 

 new species in Mrs. Gray's Collection ; one shows the whole eight 

 plates in serial position, though they are not all perfect and most are 

 in the condition of internal casts with portions of the external 

 impressions of the plates here and there preserved. In all cases the 

 shell is absent, and a cavity represents the space which it occupied 

 before it decayed or was dissolved and removed. Two other specimens 

 possess the first plate followed by several others in order, and there 

 are examples of two or more associated middle plates. 



Two nearly complete specimens of the whole mollusc exhibit- 

 a mass of spicules partly filling up the cavity of the body for its whole 

 length and slightly extending below and outside the inferior margins 

 of the plates, suggesting that the ' girdle ' was dependent or incurved 

 on each side instead of horizontally extended. The spicules occur as 

 hollow casts in a densely felted mass, and must be interpreted as of 

 the same nature as those developed round the marginal girdle of some 

 living species of Chiton. 



The minutely punctate or poriferous condition of the plates is 

 clearly seen with a hand-lens in all the external impressions, and from 

 the fact that the surface of the internal casts also shows a similar 

 granular appearance, it may be concluded that the pores pierced the 

 shell-substance. If this interpretation is correct, we may probably 

 regard the pores as corresponding to the ' micropores ' and ' megalopores ' 

 in living forms through which the network of nerves ('aesthetes') 

 reach the sense organs on the outer surface such as Moseley 1 and 

 many later zoologists 2 have described. 



According to Fischer's 3 classification this new species should be placed 

 in the subgenus Eochiton of his genus JSolochiton, Plate (op. cit., p. 384) ; 

 he considers that all the Palaeozoic Chitons belong to the Lepidopleurus- 

 stage of the group. The nearest affinities of the Girvan form are with 

 Helminthochiton Griffithi, Salter, 4 which it much resembles in general 

 shape and proportions. McCoy 5 in his figure and description of the 

 latter species reversed its position, his cephalic plate being in reality 



1 Moseley, Ann. Mag. Nat. Hist. (5), xiv, p. 141, 1884. 



2 L. H. Plate, Zool. Jahrb., Suppl. v (1901), Th. C, p. 498, and references. 



3 Fischer, Manuel de Conchyl., p. 877, 1887. 



4 Salter, Q.J.G.S., vol. hi, p. 48, fig. 6, 1847. 



5 McCoy, Syn. Silur. Foss. Ireland, p. 71, pi. v, figs. 5a-e. 



