PACHYLASMA GIGANTETJM. 479 



are added to a little above the line of attachment of the opercular mem- 

 brane ; their summits are very oblique. The sheath has its basal edge 

 slightly hollowed out. 



Basis, solid, calcareous, very irregular, and of variable thickness. 



I have, under the Genus, alluded to the structure of the rostrum : in 

 one shell, the basal diameter of which barely exceeded -joth of an inch; 

 the compound rostrum (being -g-^ths of an inch in height), had its 

 basal margin (being ^^ths of an inch in width) rendered deeply 

 sinuous (see PI. 19, fig. 5 b) by two indentations, corresponding with 

 and caused by two notches at the top of the valve. These two notches 

 extended down barely T ^o tn °f an ^ ncn (strictly T -4__ths) from 

 the summit and then disappeared ; so that when the shell was under 

 j-^th of an inch in height, (only one distinct zone of growth having 

 been formed), the now compound rostrum consisted of three separate 

 compartments ; and there were eight compartments altogether. Of the 

 above three little compartments, the middle one, or true rostrum, had 

 large alee, which could be most distinctly seen, extending on both sides, 

 under the little rudimentary rostro-lateral compartments. These latter 

 overlapped the compartments on both sides of them, as in all the 

 ChthamalinaB. 



Mouth : labrum bullate, with no central notch ; nor is the inner 

 fold of the labrum, forming the supra-oesophageal cavity, thickened, as in 

 Balanus : minute muscles run from this inner fold straight back to the 

 cavity formed by the outer bullate fold : the crest of the labrum is 

 hairy, with a row of the minutest bead-like points or teeth. Palpi, 

 small, broad, placed almost parallel to the sides of the mouth, with 

 their apices not nearly touching each other. Mandibles, with three 

 large nearly equal-sized single teeth ; the whole inferior angle strongly 

 pectinated. Maxillce, small, with a broad, square notch beneath the 

 two or three great upper spines. 



Cirri : first pair short, with the rami equal in length. Second pair, 

 with the anterior ramus having broader segments than those of the 

 posterior ramus, and with all the segments, except the few uppermost, 

 thickly covered with spines ; the posterior ramus has rather less than 

 half the segments thickly covered. Third pair, very slightly shorter 

 than the sixth pair ; anterior ramus with the lower segments, less than 

 half of the whole in number, thickly covered with spines ; posterior 

 ramus with only the lowest segments, about one fifth of the entire num- 

 ber, thickly covered ; the other segments of these two rami, and the 

 upper segments of the posterior ramus of the second pair, closely 

 resemble in the regular arrangement of their spines in pairs, the three 

 posterior pairs of cirri. The pedicel of the third pair supports nume- 

 rous, irregularly scattered bristles. The segments of the sixth pair bear 

 four or five pairs of main spines, with a few intermediate spines. 



Caudal Appendages. — Multiarticulate, narrow, tapering, situated on 

 each side of the anus : each segment has two little tufts of spines on 

 each side of its upper edge. These appendages are about one third 

 longer than the pedicel of the sixth pair of cirri: in a specimen, in 

 which the rami of the sixth pair had twenty- three segments, the caudal 

 appendages had nineteen segments. 



