9 



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 (Pis. III. & IV. 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 fi.ve large articulate, externally cUiate, lameUiform appendages (ix-xiii in 

 all the figures), each representing a pair, more or less confluent along the median line. 

 The similarly shaped appendage (viii in Pis. II A. and IV.) is usually regarded as the 

 foremost of this series ; it supports the genital outlets, which are situated on the dorsal 

 siu'face of the basal confluent segments (PI. IV. figs. 6 & 8, p). It consists of three 

 joints, of which the thu-d retains the primitive parial distinction, and supports a small 

 appendage, or fourth joint (ib. fig. 6,4). On the outer or ventral surface two oblique 

 lines mark off a small median portion of the third segment. On the inner or dorsal 

 surface the genital outlets are seen atp, and the insertions of the levator muscles at 

 m 11 : 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-XIII in PI. II. 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 lameUse (PL V. 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 lameUiform appendages are commonly termed ' opercular,' 

 as they cover the space into Avhich the genital apertures emit the products from the 

 inner or upper surface of such appendages. But each of the succeeding pairs are equally 

 ' opercular,' inasmuch as they closely overlap each other, shutting in the gills : the 

 marginal slits, defended by a fringe of cUia, allow the sea-water to filter through to 

 the branchiae, 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 (' pleou ' 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 ilattened or slightly excavated surface *. The ridges are 

 roughened with short retroverted spinules. The base of the tail has three prominences, — 

 an upper fulcral 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, 

 developed within the semicircular lower excavation, receiving the condylar part of the 

 base of the tail-spine. 



• The grounds for inferring a confluence of ' pleonal' eegments forming the basal part of the spine will be subse- 

 quently given. 



C 



