GENUS CHTHAMALUS. 453 



it is hairy, and in some pectinated with short spines. The 

 palpi are of moderate size. The mandibles have from three 

 to five main teeth, the number sometimes varying even in 

 the same species : the lower teeth are either plainly double 

 laterally, or very obscurely double, or to all appearance quite 

 single : a rather large lower portion of the mandible is finely 

 pectinated. The maxillae are always notched under the two 

 or three large upper spines: the notch bears some fine 

 spines : beneath the notch there are some large spines, and 

 at the inferior angle some smaller ones. 



Cirri. — The first and second pairs are always very short 

 compared with the four posterior pairs. The rami of the 

 first pair are slightly unequal. The third pair, in length 

 and arrangement of the spines, very closely resembles the 

 three posterior pairs ; in C. intertextus, however, the few 

 basal segments, chiefly on the anterior ramus, are thickly 

 clothed with bristles, like the segments of the second cirrus. 

 In C. antennatus (PL 29, fig. 2), the anterior ramus of 

 this same third pair is usually (one single specimen being 

 excepted) much elongated, having at least twice as many 

 segments as the posterior ramus, but the number is vari- 

 able; and these segments, either all, or only the upper 

 ones, instead of having their spines regularly arranged in 

 pairs, in a double row, are surmounted each by a circle of 

 spines: I suspect that these elongated rami of the third 

 cirrus act as antennae. It can hardly be an accidental coin- 

 cidence, that certain genera, as Lysmata and Pandalus, 

 amongst the Macrourous Crustaceans, have the same leg 

 (homologically the second thoracic limb) elongated and 

 antenniformed. Certain varieties of C. stellatus and cirratus, 

 also, have the anterior ramus of this same third cirrus con- 

 siderably elongated. We are thus reminded of the remark- 

 able variability in the numbers of the segments, and in the 

 arrangement of their spines, in the cirri of Tetraclita porosa ; 

 in that species, however, it was chiefly the terminal seg- 

 ments of the posterior ramus of the third cirrus which were 

 so highly variable. The three posterior pairs of cirri in 

 Chthamalus support from three to five pairs of main spines 

 on each segment, the number often varying in the same 

 species, with some minute intermediate bristles. The 



