THELEPUS TKISERIALIS. 177 



No member of the family lias been the subject of more ambiguity or of a greater 

 expenditure of time and labour than this form, a result perhaps partly due to the 

 variability of its characters and the fineness of the distinctions between it and 

 neighbouring species. It agrees with the northern Thelepus cincinnatus, Fabr., in having 

 two pairs of simple branchiae and in occasionally presenting the minute papillae at the 

 dorsal end of the hook-rows 4 — 7, but it shows an additional tooth above the main fang of 

 the hook, and the base is somewhat shorter and broader, the stud above the prow projecting 

 nearly in a line with the trend of the anterior outline, whilst the stud in T. cincinnatus 

 makes a smaller angle with the anterior outline. The process beneath the stud is generally 

 larger. T. cincinnatus, var. andreanm, is distinguished from T. triserialis, Grube, a southern 

 form characteristic of the Channel Islands and the Mediterranean, by the occurrence of 

 three sets of gills in the southern type, by its hooks having a longer base and often only 

 a single tooth above the main fang, though a second is occasionally seen. Streblo- 

 soma Baird/L differs in having much longer setigerous processes, three pairs of gills, 

 hooks which have the stud set at a different angle, and with a prolongation of the anterior 

 outline in a line with that above the stud. Two conspicuous teeth occur above the 

 main fang. From Thelepus setosus, De Quatrefages, T. cincinnatus, var. andreance, is 

 separated by its two pairs of gills, and by the commencement of its hook-rows on the 

 fifth segment, whereas those of T. setosus begin on the third segment. 



Yet after all these distinctions the feeling remains that the precise degree of 

 variability in each has never been fully studied, and that the environment of each 

 may be the cause of its special divergencies. All that can be done at present is to 

 define such variations around each centre or supposed species. The variations of the 

 hooks of the widely distributed Thelepus setosus, De Quatrefages, as recently shown by 

 Prof. Fauvel 1 (1916), afford an excellent argument for caution in dealing with forms so 

 intricate. Future work may show that the northern Thelepus cincinnatus of Fabricius 

 may be the centre of the various forms hitherto described as species, for the presence or 

 absence of a pair of gills, or the curves and processes of a hook or even the modifications 

 of the nephridia are not necessarily in themselves features of fundamental value. 



Von Marenzeller (1884) mentions that he has found, as Grube previously noted, 

 specimens in which the bristles did not reach the posterior end. He also observes 

 that Phenacia ambigrada and P. retrograda of Claparede refer to young examples. 

 He appears to group all the varieties under this species. In all probability the author 

 refers to the same form as here described, both from the structure of the hooks and 

 other particulars. 



Crawshay (1912) gives an account of the structure of the hooks and their varia- 

 tions at Plymouth, and he probably refers to this form, which differs from the northern 

 T. cincinnatus. 



2. Thelepus triserialis, Grube, 1885. Plate CXXVa, figs. 7 — 7 b — bristles and hook 



Specific Characters. — General aspect of the cephalic region and body similar to that 

 of T. cincinnatus, but three pairs of branchiae occur, viz. on the second, third and fourth 



1 < Arch. Zool. Exper./ t. lv, p. 465, text-figs. 3—6. 



192 



