NATURAL HISTORY SOCIETY OF GLASGOW. 101 



slopes obliquely from the apex to the margin ; flanks of the plate 

 rather pinched-in. Exposed surface proportionately larger than in 

 the other plates, and with a few coarse radii from the apex round 

 the posterior end of the plate; ridges variable in number, and 

 alternately larger and smaller. 



The exposed surface of all the plates is microscopically granular. 



Obs. — The large proportion of inserted or covered surface of these 

 plates, as compared with the exposed, and the nature of the orna- 

 mentation, will clearly place this form with Chitonellus rather than 

 Chiton. At any rate this arrangement will be preferable until some 

 further sub-division of the genus Chiton is proposed for the 

 accommodation of the palaeozoic species of the family. The refer- 

 ence to Chitonellus is borne out by the absence of apophyses, and 

 by the form of the posterior plate. The latter is quite chitonellid 

 in its elongated narrow outline, and so far is comparable with that 

 of Chitonellus Youngianus, Kirkby, whilst the arched nature of 

 the plates shews a departure towards those known as Chitonellus 

 subquadratus, K. & Y. 



The figure of the posterior plate, given by de Kyckholt,* as 

 that of CJdtonellus cordifer, de Koninck, has much resemblance to 

 that of the present species. 



Mr. Kirkby, who has examined specimens, believes this form to 

 be undescribed and to require separation. So far as my own 

 researches have gone it is so, and I have, therefore, much pleasure 

 in associating with it Mr. Kirkby's name, thanking him, at the 

 same time, for the assistance he has always kindly rendered me. 



Locality and Horizon. — Law Quarry, as before. 



Collections. — J. Bennie, J. Armstrong, J. Smith, &c. 



Cliitonellus Youngianus, Kirkby. — (PL II., figs. 23, 24.) 



C. Youngianus, Kirkby, Trans. Geol. Sou. Glasgow, 1865, ii., 

 p. 14, t. 1, f. 2. 

 ,, Kirkby and Young; Geol. Mag., 1867, iv., 



p. 341, t. 16, f. 2-4. 

 Obs. — A fine posterior plate of this species has reached me from 

 the cabinet of Mr. Armstrong, in which all the details are exceed- 

 ingly well shewn. The plate agrees with the description given by 

 Kirkby and Young, except that the exposed portion of the plate is 

 rather more pointed in front than is represented in the published 

 * Bull, de VAcad. R. de Bruxelles, 1845, xii., pt. 2, t. 4, f. 10. 



