8 



The flat vmder surface of the digging-blade (Pis. II., III. c) is broadest at the point 

 of most resistance, viz. at the foremost part of the curved edge. From this part the flat 

 tract extends backward to its hinder border, which forms a pair of bold curves, arching 

 outward and backward from the hindmost point, which is in the mid line, and in the 

 form of a retroverted spine, supported by a vertical buttress-like ridge. The under 

 hard chitine (Pis. II., III. A') rises rapidly from the curved hind borders of the flat part 

 of the blade toward the softer chitine, forming the arched or vaulted roof and sides 

 of the cavity concealing the mouth and its environing pairs of jaw-feet as the crab is 

 viewed from above. Into this vault will slip or be pressed the sand or mud displaced 

 by the forward and downward thrusts of the spade ; and the burrower will have the 

 advantage of the additional firmness so given to the cephaletron as a point of resis- 

 tance to the fulcra and muscular powers then acting from it upon the thoracetron and 

 the telson, drawing them in, and fixing the latter in the position in which, like an 

 ' alpeu-stock,' it can best help forward in the renewed locomotive act, when the muscular 

 powers and entapophysial fulcra combine their mechanism to again move forward and 

 press down the great cephaletral spade. 



Meanwhile, in the loosened mud or sand so driven back into and filling the under 

 hollows or vaults, the six pairs of jointed cii-cum-oral appendages are busily at work 

 sifting the displaced material in quest of whatever organic matter may be included 

 fit for food. 



Save the groove extending along the posterior facet, all traces of the segmental 

 constitution of the cephaletron are obliterated in its growth, and are recognizable, 

 externally, only through the appendages and sense-organs of this main division of 

 the body. 



In the thoracetron the segments are indicated not only by the appendages beneath, 

 but by the pairs of entapophysial pits above, and by the notches and their articulated 

 spines on each side. These spines are the ' epines laterales ' of Van der Hoeven * 

 {m i-B, PL I. fig. 1) ; the fixed spinous productions (ib. n, n) of the borders of the 

 alveoli of m 1-6 are termed by Van der Hoeven the 'teeth' f. The hindmost of this 

 series (PL I. fig. 1, n t) terminates the lateral border, and projects beyond the posterior 

 concavity for the articulation of the tail-spine (c). 



The trilobitic accentuation of the upper surface oiLimulus is continued on to the thorac- 

 etron by the pair of longitudinal depressions beginning where those of the cephaletron end, 

 and extending about halfway along the thoracetron : in these depressions are the series of 

 narrow oblong pits, commencing with the pair (ib. h) in the coalesced segment at the back 

 of the cephaletron, and which, as they indicate the places of attachment of the entapophyses 

 projecting from the inner surface, I term « entapophysial :' there are six in each series 

 {ib. fig. 2, i i-i e) in the thoracetron proper, seven with those of the opercular segment, h. 

 The intermediate rising is sul^angular, with a spine at the fore part of the ridge, a second 

 at the part where the longitudinal depressions cease, and a third at the hind end of the 



* 0/). cif. p. 11. 



t " Nous donnerons le nom de dents aux opines immobilos, et nommerons simploment ejyines latemles celles qui 

 sont articulees." — Ib. p. 11. 



