100 SIK C. ELIOT OX NIDIBRAN'CUS [May 17, 



BoRX'ELLA. 



The members of this genus are sleiidei', elegant animals, having 

 on either side of the back a I'ow of cerata mostly divided into 2-4 

 bi-anches and bearing gills. On either side of the month is a 

 compound tentacular process consisting of a number of conical 

 tubercles set in a sort of rosette. Over the head are a pair of 

 laige oi'gans called in the following descriptions for brevity's sake 

 rhinophore-sheaths, but appai-ently foi'med by a fusion of the true 

 i-hinophore-sheaths with a pair of cerata. The paii- of cerata after 

 these organs are called the first pair. The vent is latero-dorsal 

 between the first and second pair of cerata. The buccal mass is 

 not large, but veiy muscular ; besides the jaws and radula, there 

 is also a labial armatui'e of scales. The radula consists of a median 

 tooth, roughly triangular, either smooth or denticulate, and a few 

 (9-19) smooth hamate latei'als, bent somewhat forward. The 

 innermost are genei-ally veiy small, and the size increases towards 

 the outside of the row. There are two stomachs, of which the 

 second is aimed with spines, and two accessory livei'S, besides the 

 main mass. As a ride ramifications of the liver enter the cerata, 

 but thei'e is some irregularity in this respect. The hermaphiodite 

 gland lies on the liver ; the piajputium is smooth or armed with 

 spines. 



There is considerable difficulty in dividing the genus into species. 

 The colour presents little vaiiety, being in all the known forms 

 whitish yellow, with a red or yellow reticulation on the back. 

 On the other hand, thei-e is some vaiiety in both the extei-nal 

 and inteiiial oigans. The number of the cerata and their sub- 

 divisions appeai-s not to be specifically characteristic, but to increase 

 with age, and is not always the same on the two sides of the 

 body. The ramification of the liver may be present or absent in 

 the same species {B. excepta ; see Bergh's two descriptions), and, 

 when present, may not extend to all the cerata. The armature of 

 hooks on the pra?putium may also be present or absent in the 

 same species {B. diyitata and B. arhorescens ; see Bergh). IS^ine 

 species are recorded, but B. hermanni Angas, caledonica Crosse, 

 adamsii Gr., semperi Crosse, and hancockana Kel., will hardly 

 prove valid, foi- even if they represent specifically distinct forms 

 they are insufficiently charactei'ised. Of the remaining species 

 B. calcarata Morch, from the West Indies, is distinguished by 

 having appendiculate rhinophore-sheaths and smooth median 

 teeth. The Indo-Pacific foi-ms fall into two groups — the one 

 represented by B. digitata, with a single process behind the rhino- 

 phores, cerata divided into rather long eiect fingers, and median 

 teeth with faint denticulations ; the other by B. excepta, with 

 several processes behind the rhinophores, small fingeis on the 

 cerata protecting the external branchife, and much more distinctly 

 denticulate median teeth. Whether B. digitata and B. arhorescens 

 are really distinct is discussed below. B. simplex, n. sp., is 

 certainlv a separate species, unless it is a monstrosity. 

 [18j 



