294 E. Hull — General Relations of the Drift Deposits. 



them at least — have a shelly covering of considerable thickness, it 

 is not unreasonable to assume that their locomotory appendages were 

 calcareous, or at least chitinous and of sufl&cient strength to enable 

 them to crawl upon or burrow in the mud at the bottom of the 

 Palaeozoic seas in which they dwelt. 



With regard to the buccal apparatus, we know that the hypostome 

 or lip-plate is commonly present in most genera of Trilobites. This 

 plate agrees most closely with the hypostome in Apus (see Geol. Mag,, 

 Vol. II. PL XL p, 401), a similar plate exists also in most of the 

 Isopods. That its presence indicates the possession of maxillas, I 

 have no more doubt than I should feel in asserting the presence of 

 jaws within the lips of a mammal. Looking at the group as a whole, 

 it would also, we think, be a safe prediction that their branchiae 

 were abdominal, the great caudal shield offering a strong analogy to 

 the tail-plate which, in the modern Isopods, covers those organs. 



In the foregoing remarks my object has been mainly to show the 

 reasons why it is desirable to adhere to the original interpretation of 

 the appendages in the Trenton Asaphus, as proposed by Mr, Billings, 

 rather than to adopt the conclusions of Prof Dana, whose opinion, 

 however, upon living Crustacea I hold in the highest estimation. 



The patient examination and slicing of more specimens appears 

 in the mean time to be the most hopeful method to adopt in order 

 conclusively to settle the question of the structure and appendages 

 of the Trilobites, 



EXPLANATION OF PLATE VIIL 



Fig. 1. Asaphus platycephalus, Stokes. — Underside, showing the leg's. (Copied 

 from figure in Quart. Journ.Geol. Soc, 1870, vol. xxvi. pi. xxxi.,fig. 1.) 0. Cephalon, 

 to which is united the hypostome or lip -plate {h), followed by eight free thoracic 

 segments, indicated in outline, bearing eight pairs of appendages on their underside. 

 These again are followed by the pygidium or tail (P) composed of the abdominal 

 somites welded together, and probably bearing the branchige upon the underside, to 

 which the points (br) seen in the iigure, may perhaps relate. 



Fig. Iff. Ideal section of one of the thoracic segments, ^ = tergum, s=sternum, two 

 legs a a, are seen beneath, articulated to the sternum near the median line, and having 

 their muscular attachments m m, (and apodemata ?) on the underside of the tergum (t) . 



Fig. 2. Underside of abdomen in Norway lobster. Nephrops Norvegicus, Leach. 

 s s, sternal arches ; m m, membranous interspaces, a 1. 1st, thoracic somite, with 

 its modified pair of appendages still attached to sternal arch, b b, points of attachment 

 for abdominal appendages which have been removed, however, to exhibit the cal- 

 cified arches of the sternum more clearly ; ep =epimera ; th = thorax. 



Fig. 2a. One of the abdominal somites detached, <= tergum; s= sternum; 

 ep = epimera; ex = exopodite ; en=endopodite. 



II, — Observations on the General Eelations of the Drift 

 Deposits of Ireland to those of Great Britain. 



By Edwakd Hull, M.A., F.E.S., F.G.S., 

 Director of the Geological Survey of Ireland, 



THE valuable paper by Professor Harkness on "The Middle 

 Pleistocene Deposits of Britain," published in the pages of 

 this periodical,^ was a completely successful attempt to synchronize 



1 Geol, Mag,, Vol, VI., p. 542, 



