Mr. Alder on the Genus Polycera. 



537 



XXXVIII.— Observations on the Genus Polycera of Cuvier, 

 with Descriptions of two new British Species. By Joshua 

 Alder, Esq. 



[With a Plate.] 



In consequence of the small number and rarity of the species 

 constituting the genus Polycera of Cuvier, its characters and 

 limits are at present very imperfectly understood. I shall, 

 therefore, make no apology in bringing under the notice of 

 naturalists two or three species belonging or allied to this 

 pretty genus, which have lately occurred to me on the coast 

 of Northumberland, near Tynemouth and Cullercoates, in the 

 hope that the more detailed descriptions and figures which I 

 shall be able to give, from having examined the animals in a 

 living state, may furnish materials for estimating the value of 

 those characters on which a generic distinction ought to be 

 founded. 



Cuvier describes the genus as having the branchial plumes 

 accompanied by a pair of membranous lamellae capable of 

 covering and defending the branchiae in cases of danger*. 

 Though the number of these appendages is stated by Cuvier 

 to be two, it seems doubtful whether the character of the ge- 

 nus should be thus limited. Rang, who had the opportunity 

 of examining some exotic species, does not confine his generic 

 character to that number, but merely says in his description of 

 the branchiae, that 66 quelques appendices membraneux les en- 

 vironnent." Without giving any opinion in this matter, it 

 will suit my convenience on the present occasion to adopt the 

 latter and more enlarged view of the characters of the genus, 

 for the purpose of including in it two new species about to 

 be described, which, though they differ in some other respects 

 from the tj^pical form, I feel reluctant in the present imper- 

 fect state of our knowledge to raise to the rank of genera. 

 They constitute, however, two very marked divisions, the 

 characters of which may probably afterwards be found of suf- 

 ficient value to constitute generic distinctions. Until lately 

 the only known British species of this genus was the Doris 

 flava of Montagu, to which Dr. Fleming has added, with some 

 doubt, the anomalous Doris penniyera of the same author. 

 Neither of these species appear to have been recognized by 

 later observers. Very recently the indefatigable researches of 

 Mr. Thompson of Belfast have brought to light two other 

 British species of this genus, (if, indeed, one of them be not 



* The use here attributed to these appendages appears to be in part, at 

 least, imaginary. Such an use is not and cannot be made of them by some 

 of the species ; their position and small degree of motion preventing their 

 being so applied. 



Ann. § Mag. N. Hist. Vol. vi. z 



