536 THE VOYAGE OF H.M.S. CHALLENGER. 



fingers of the chela, but they are only subservient to the larger organ, while in this case 

 the chela is rudimentary. The presence of long calcified tendons within the propodos 

 is strong evidence that the dactylos is a movable joint, and the movable power that it 

 possesses must have its usefulness increased by the presence of these long hairs. It 

 appears to offer an example of how a simple and apparently useless organ may by 

 adaptation become converted into one of importance. 



The third pair of pereiopoda is longer than the second and more robust, and terminates 

 in a short, single, sharp pointed unguis ; the carpos and the meros at their distal 

 articulation on the upper surface overlap the next succeeding joint. The fourth pair of 

 pereiopoda resembles the third in proportion and form. The fifth is shorter and terminates 

 in a stunted dactylos. 



The pleopoda are biramose and subequal. 



The lateral plates of the rhipidura are longer than the telson and possess a small 

 diaeresis. 



The telson is long, slender, and tapering. 



Observations. — This, our only specimen, is small and semi-transparent. It was dredged 

 in the same locality as Synalpheus, Paralpheus, Alpheus avarus, and that which I 

 have thought resembles Alpheus acuto-femoratus. The form of the chela in this 

 species so greatly corresponds in appearance with that of the right or smaller one in 

 Synalpheus falcatus that I at first thought it must be a younger form of that sj^ecies, 

 or at least of some species of the same genus, but an examination of certain parts renders 

 this most improbable. In Synalpheus falcatus the dorsal surface of the carapace is 

 elevated into a carina, which decreases in intensity on the pleon, but still remains con- 

 spicuous, although more as a dorso-lateral compression than as a distinct keel. In 

 Cheirothrix parvimanus the carapace is not elevated and the dorsal surface is smooth 

 and even. In Synalpheus the first pair of antennse has a tendency to divide into two 

 at the extremity of the stouter flagellum, which does not appear to be the case in 

 Cheirothrix. The second pair of antennse in Synalpheus has the scaphocerite sharp 

 pointed, and the outer styliform process separated from the inner foliaceous plate by nearly 

 half its length. In Cheirothrix it is broad and ovate, rigid on the outer margin, and 

 connected with the foliaceous plate to near its distal extremity, where it terminates in a 

 short tooth. The first pair of gnathopoda differs in form, and also apparently in 

 structure, since in Synalpheus the position of the dactylos varies from that in Cheirothrix. 

 and in the latter genus the second pair at its distal extremity is tipped with short spinules, 

 while in Synalpheus it terminates in two points and a few fine hairs. 



The generic distinction between this species and those of Alpheus or its allies must 

 rest upon the importance of the change in the form of the second pair of pereiopoda. 



In all species of Alpheus and its related genera the second pair of pereiopoda is as 

 long as, or longer than the third pair, it is slender and has the carpos more or less 



