nritish .S)'ecjM o/"Sii)lioiiostoina. 11' I 



•V. (iffinis (spt'fiiiu'Hs from S. diplochaitM (Nuples 



l>iiiar(I). 8pc'ciiuou8). 



Length I'O 10 millini. Longtli 70 SO inillim. 



]tii-adt)i 2 •'{ inilliiii. |{|-i'ii<ltli 7 iiiilliiii. 



SuLMiK-iits ."JO 47. 8»'^:iiR'iits .'{'.» 40. 



GiVb 40-r)0. (Jills sO-100. 



Cephalin liri.Htle.< (^0 80. Coplialir' Eristics 200 or more. 



Notopoiiial hristk's 4-r>. Notojxuliiil b^i.•^t!t•.^ l:i-14. 

 Neuri>podiuiii Mith 1-2 honks and NouriniiMliiiiu with 4-5 hooka and 

 G .'iiniple bristles. G simplti bristled. 



Thus, nparl from size, the differences chiefly consist in the 

 disparity in numbers of the various append:i<^e.s. These 

 distinctions are somewhat vitiated hy tiio fact that 8t. Joseph 

 is incUned to rc^jjard Chlorirma DiijariJini'i^ Qf.^'-> ^^ the 

 young of S. affiuis, and it differs in tlie iliniinished number 

 of segments, gill.", and cephalic bristles. It is obvious that if 

 in i>. affiiiis these structures increase in number througliout 

 life, they may also do the same in S. diplochi'Uos, and the 

 young stages of the latter may thus resemble tiie adults 

 of S. affihis. In fact tiiis is precisely what the Millport 

 specimens show. St. Joseph, however, furtlier seeks to 

 difterentiate the two species by tiie minute characters of tiie 

 bristles, and states that these are of much importance in 

 distingui-shing the .species in the genus. The special points 

 upon which he relies are the following : — 



In S. affiuis the distance between the successive annuli of 

 the ce|)halic bristles is 0*025 millim. Tiie hooks have the 

 following structure : immediately below the hooked region 

 there are %(}> annuli, the intervening spaces gradually 

 increasing, but never exceeding 0*02 millim. ; there is then a 

 long gap of 0"J millim. in length, followed by 10-20 annuli 

 separated by spaces of about 0*02 millim. 



In IS. diplocliu'Uos the distance between the successive rin"-3 

 of the cephalic bristles is only 0*016b millim. In the hooks 

 the hooked region is less curved ; it is followed by 50-60 

 annuli placed very clo.se together, then by 4 annuli separated 

 by distances of O"048 millim., finally by 50 closely aggre- 

 gated annuli. 



As to my own observations, tlie Plymouth specimens agreed 

 very closely with St. Joseph's description of S. affinis. The 

 length varied from 15-25 millim., with a usual breadth of 

 2 millim. The number of segments varied from 39—45, and 

 there were about 40 gills. In regard to the cephalic bristles 

 there were some slight differences ; usually numbering ab)ut 

 80, in one case at least a very careful count gave over 100 

 though some of these were short and small. As to the 



