384 MEMOIES OF THE NATIONAL AOADEMY OF SCIENCES. 



individuals found iu them beloug to this variety. The color variations of this form have already 

 been giveu iu section i. 



The rostrum is sometimes wanting, as iu the individual from which Fig. 11, PI. xxii, was 

 drawu. This variation has been noticed iu other species and is interesting, siuce the absence of 

 the rostrum is a constant character in a closely related series of forms, which are placed by Dana 

 in a separate geuus (Betwus). These variatious indicate that the uniform presence or absence of a 

 rostrum is a specific aud not a generic character, as has already been shown by Iviugsley (29). 

 The structural poiuts of diflereuce between the loiKjicarpus and the other form lie chietly iu the 

 antennae and first pair of walking legs. These may be seen by a comparison of Figs. 11, 13, 18, 

 PI. XXII, and Fig. L', PI. xxiv, with Figs. 4, 8, PI. xxiii, aud Fig. 3, PI. xxiv. 



In the first pair of auteun:e the aural spine (Fig. 11, PI. xxii) is scarcely more than half the 

 length of the first segment of the stem. It is blunt aud somewhat ovate iu shape, as seeu from 

 above. 



(2) In the other form (var. brevicarpus) the aural spine (Fig. 4, PI. xxiii) has a dittereut shape, 

 and is relatively uearly twice as long. In this case it extends beyond the first segmeut to two- 

 thirds the leugth of the second. The secoud or outer auteuua of the lonywirpnn is armed with two 

 spines at its base (Fig. 11, PI. xxii) ; an inferior and outer basal spiue, and a sligiitly longer one, 

 the squamous spine, articulated to the joint carryiug the latter. There is uo scale. The basal s[)iue 

 is rather more than one-half the leugth of the antenual stalk. There may be present a small 

 tubercle on the upper surface of the segment beariug the basal spine, near the articulation. 



In variety hreviearpvs (Fig. S, PI. xxiii) the squamous spuie is stout aud reaches nearly to the 

 end of the antenual stalk. There also spriugs from its inner and proximal margin an elongate 

 plate or scale, the inner free edge of which is fringed with plumose setre; scale not quite as long 

 as spiue. The inferior basal spine not one-half the length of the squamous spiue. There is a rounded 

 or pointed tubercle over basal spine near the joint. 



The small chela of the first pair of thoracic legs of the lougicarjms (Fig. 2, PI. xxiv) is short 

 and broad. The finger ends in two or three horny teeth or prongs, which interlock those of the 

 opposing thumb. The dactyle bears on its outer surface a tuft of peculiar hairs. The latter are 

 finely serrate and have beut or hooked tips. The carpus is relatively very long, quite as long as 

 the palmar portion of the propodus. 



In the breriearpus the small chela is long aud somewhat narrower (Fig. 3, PI. xxiv). Tips 

 of fingers usually simple, but sometimes notched; the peculiar tuft of hairs is wanting. Carpus 

 relatively short ; about one-third the length of the palm. 



The large chela of the longicarpus may also difler noticeably from the brevicarpus type. (Com- 

 pare Fig. M, PI. XXIV with the figures on PI. iv.) Fig. 8 represents a common form of this append- 

 age. The propodus is long, cylindrical, slightly twisted, very smooth, and polished; ends above 

 dactyle in a short spine aud below iu a rudimentary thumb with claw-like tip. Dactyle overreaches 

 proijodus, and its inner margin is not concave, or but slightly so. 



These two forms, difleriug in the particulars just mentioned, would doubtless be considered 

 as two distinct species if only these facts were known. A prolonged study, however, of a large 

 number of individuals, collected both iu sponges and from porous rocks on a number of reefs, has 

 i-esulted iu the discovery of a complete series of intermediate links. These conuecting forms sug- 

 gest a number of important questions relating to the causes and significance of variation. 



By far the greater number of individuals of this species have the characteristics of the two 

 varieties just described, but about five per cent of the collection made at diflereut points near Nassau 

 present intermediate characters.* 



More fully stated, the noticeable points of variation are as follows: (1) The relative length of 

 the autenuular stalk and aural spine; (2) the lengths of the antenual spines relative to each 

 other and to the peduncle of the antenna; the presence or absence of a squame or scale; (3) 

 the character of the dactyle and propodus of the small chela of the first pair of pereiopods ; (4) 

 the length of the carpus of the small cheliped; (5) the general shape and character of the large 



* While the species live, as a rale, in the interior of the green aud brown sponges, a few undersized individuals 

 may be found, by careful searching, among the loose blocks of porous coral which are scattered over the reef, and it 

 frequently happens that these iudividualt possess intermediate characters between the two varieties just described. 



