the median than the lateral ridges. Tliese depressions, with the median and lateral eleva- 

 tions, give the trilobitic character to the carapace or dorsal wall of the cephaletron of 

 Li/inuliis. Each longitudinal ridge terminates by a short spine posteriorly. They are 

 much produced in the carboniferous Limuloids, e. g. Presticichia, as in some Trilobites, 

 whence they have received the name of ' genal,' or ' cheek '-spines (in Trinuclevs, e. g.). 



The hind vertical part or border of the dorsal wall of the cephaletron is divided into 

 a median transverse tract of equal extent with the fore border of the thoracetron there- 

 with articulated, and two lateral oblique tracts passing outward and backward to the 

 hinder angles of the cephaletron, which are usually spiked (PI. V, a"). The transverse 

 posterior tract is vertically thick, as if truncate. At the middle third, included by the 

 hind deepened ends of the lateral longitudinal dorsal furrows, the upper border rises 

 like the slopes of a low roof on each side to the median longitudinal dorsal ridge, here 

 raised into a spine. The border below this is arched for the reception of the fulcral 

 levator-process of the thoracetron, which passes beneath the arch. The joints of the 

 thoracetron with the cephaletron are at the straight lateral tracts on each side the convex 

 process, entering the arch and at a lower level. Consequently a power tending to pro- 

 tract or pull forward the process, being opposed by the joints or centres of motion below, 

 raises the thoracetron upon those centres, and brings it to a line or to a level with the 

 dorsal plane of the cephaletron, or even raises it to a slight angle therewith. At the 

 lower part of the transverse hinder tract, outside the piers of the arch, are the pair of 

 small crescentic pilose depressions (PL I. fig. 1, h) corresponding with the origins 

 of the entapophyses within. Pi*om the posterior spine (ib. k), teruunating the lateral 

 ridge of the dorsal surface of the cephaletron, a lower ridge passes downward and out- 

 ward, to be lost in the lateral tract of the hind border. The indentation or groove 

 between the ridge and border is continued along the thick transverse hind part of the 

 cephaletron, defining the portion below, which articulates with the thoracetron. In this 

 groove is sitiiated the entapophysial pit (/?-). To the lower part of the so-defined hind 

 surface of the cephaletron is attached the opercular plate, or first coalesced pair of 

 lamelliform limbs (Pis. II., fig. 1, IV., fig. 1, viii). I view the groove above described 

 as a persistent indication of an originally separate segment. This segment, by the 

 entapophysial pits above and the lamelliform appendages below, belongs to the category 

 of ' thoracetral ' plates : it is cephaletral only by confluence. 



The general aspect of the cephaletron of Limuhis reminds one of a rounded spade- 

 blade, or the blade of a saddler's knife. It gives forward a digging-edge, curving outward 

 and backward to nearly twice the breadth of the following segment ; so that this can be 

 drawn along in the track delved out by the foremost one with least resistance. The hard 

 chitine (PI. II A. fig. 2, /) of the arched upper surface of the cephaletron {b) meets 

 a flat tract of the same material below (ib. c), at an acute angle, to form the digging- 

 edge [d) ; and this edge is strengthened by a low ridge, like the carpenter's ' bead,' 

 running above it, along the extent where most resistance has to be overcome, the ' bead ' 

 subsiding or falling into the edge at the hinder angular spiked ends (Pis. I. and IV. A"), 

 which terminate outside the thoracetron {ib. B), nearly halfway toward the hind end 

 of that division of the body. 



