122 Mr. J. G. Jeffreys on the Descent of Glaciers » 



and the cubital and radial joints are clavate, the former having 

 a conical, pointed process projecting at right angles from its ex- 

 tremity, on the under side, and the latter a very minute, bifid, 

 black apophysis at its extremity, in front; the digital joint is 

 small, oval, convex and hairy externally, concave within, com- 

 prising the palpal organs, which are moderately developed, not 

 very complicated in structure, and of a pale red-brown colour. 

 The cephalo-thorax is oval, convex, glossy, with slight furrows 

 on the sides converging towards an indentation in the medial 

 line ; the falces are powerful, subconical, vertical, convex at the 

 base, in front, divergent at the extremity, armed with teeth on 

 the inner surface, and have a conical tooth-like process near the 

 middle, towards the inner side, and numerous minute, pointed 

 prominences in front ; the maxillae are convex at the base, en- 

 larged where the palpi are inserted, and at the extremity, which 

 has a pointed process on the outer side, and incline towards the 

 lip, which is semicircular and prominent at the apex ; and the 

 sternum is broad, glossy, and heart-shaped. These parts have a 

 reddish-brown colour, the lip and anterior part of the cephalo- 

 thorax being much the darkest. The four intermediate eyes 

 form a trapezoid, the two anterior ones, which constitute its 

 shortest side, being the smallest and darkest of the eight ; and 

 those of each lateral pair are seated obliquely on a small tubercle, 

 and are almost in contact. The abdomen is oviform, thinly 

 clothed with hairs, convex above, projecting over the base of the 

 cephalo-thorax ; it has a dark olive hue, the under part being 

 the palest, and the colour of the branchial opercula is yellow ; 

 along the middle of the upper part there extends a series of 

 obscure, curved, grayish lines whose convexity is directed for- 

 wards ; and two indentations occur on each side of the medial 

 line, the posterior pair being rather the wider apart. 



Two adult males of this species were received from Mr. R. H. 

 Meade in June 1855, one of which had been taken in the vici- 

 nity of Burton-on-Trent, and the other at Hornsea, near the east 

 coast of Yorkshire, in the preceding year. 



XII. — Note on the Descent of Glaciers, 

 By J. GwYN Jeffreys, Esq., F.R.S. 



The different theories, propounded from time to time by so 

 many able observers of this singular phsenomenon, have been so 

 earnestly and plausibly argued, that it may be worth while to in- 

 quire if they cannot be reconciled with each other ; and, although 

 my knowledge of the subject does not enable me to do so, I 

 trust I shall not be considered presumptuous in offering a sug- 



