OF THE AMERICAN KING-CRAB. 467 
The intermediate rising is subangular, 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 
mid rising. The lateral, low and broad, convex risings exterior to the depressions 
subside where those terminate rather more than halfway toward the hind border of the 
thoracetron. 
The under surface (Pl. XXXVIII. B’) defines the cavity lodging the articulate lamelli- 
form appendages by a prominent border, within which the chitine loses density, where 
it forms the roof of that cavity. To this roof, or to the ventral surface of the thoracetron, 
are attached five large articulate, externally ciliate, lamelliform appendages (1x-x111 in 
all the figures), each representing a pair, more or less confluent along the median line. 
The similarly shaped appendage (vii in Pls. XXXVII. and XXXVIII.) is usually 
regarded as the foremost of this series ; it supports the genital outlets, which are situated 
on the dorsal surface of the basal confluent segments (Pl. XXXVIII. figs. 6 & 8, p). It 
consists of three joints, of which the third retains the primitive parial distinction, and 
supports a small appendage, or fourth joint (ib. fig. 6,4). On the outer or ventral sur- 
face two oblique lines mark off a small median portion of the third segment. On the 
inner or dorsal surface the genital outlets are seen at p, and the insertions of the levator 
muscles at m u: the articular surfaces at which this coalesced pair have been detached 
are marked r. 
The succeeding thoracetral appendages are 4-articulate, as is shown in the sections of 
ix-xi in Pl. XXXVII. fig. 1. The basal joints are confluent medianly and ciliate 
laterally, like those of the first, they having attached to their upper or dorsal surface, 
along its outer two-thirds, the branchial lamellæ (Pl. XXXIX. fig. 2). The three distal 
joints preserve their median distinction: the last joint is narrow, ovate, and projects 
beyond the lateral divisions of the broader antecedent joint. 
The first pair of confluent lamelliform appendages are commonly termed * opercular, 
as they cover the space into which the genital apertures emit the products from the 
- inner or upper surface of such appendages. But each of the succeeding pairs are equally 
* opereular, inasmuch as they closely overlap each other, shutting in the gills: the 
marginal slits, defended by a fringe of cilia, allow the sea-water to filter through to 
the branchise, and exclude the particles of sand or mud diffused abundantly, by the 
rapid action of the cephaletral limbs, through the respiratory medium during the 
burrowing procedures. 
The tail-spine (* pleon” and ‘telson,’ c in all the Plates) nearly equals in length the 
two antecedent divisions: it is three-sided, with one ridge or angle dorsal and two lateral, 
bounding the lower or ventral flattened or slightly excavated surface. "The ridges are 
roughened with short retroverted spinules. The base of the tail has three prominences,— 
an upper fuleral one, which, in the extended state of the spine, fits into the arched 
fossa beneath the back border of the thoracetron: this process receives the insertions of 
the *levatores muscles. The other two prominences form a pair of articular condyles, 
adapted to cavities completed below by a pair of prominences of the thoracetron, 
* The grounds for inferring a confluence of * pleonal' segments forming the basal part of the spine will be subse- 
quently given. 
3 R 2 
