466 PROF. OWEN ON THE ANATOMY 
XXXIX, A”), which terminate outside the thoracetron (ib. B), nearly halfway toward 
the hind end of that division of the body. 
The flat under surface of the digging-blade (Pls. XXXVII., XXXVIII. 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 
eurves, arching outward and baekward 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 (Pls. XXXVII., XXXVIII. 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 resistance to the fulcra and muscular powers then acting from it upon the thorac- 
etron and the telson, drawing them in, and fixing the latter in the position in which, 
like an *alpen-stock, it can best help forward in the renewed locomotive act, when the 
muscular powers and entapophysial fulera 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 circum-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 ‘ épines latérales' of Van der Hoeven * 
(m xs Pl. XXXVI. 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’ +. The hindmost of 
this series (Pl. XXXVI. fig. 1, 7) 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 of Limulus 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. ^) 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 ı—i 6) in the thoracetron proper, seven with those of the opercular segment, h. 
Op. at. PE ; 
t * Nous donnerons le nom de dents aux épines immobiles, et nommerons simplement épines latérales celles qvi 
sont artieulées."— 75. p. 11. 
