SOME ASCIDIANS FEOM THE ISLE OF WIGHT. 187 



78 to 85 in Neapolitan specimens^ while Heller, who also examined 

 the species in great detail, ascribes from 30 to 35 to Adriatic 

 examples. There are 30 in one of mine, 40 in the other. Herdman's 

 Ascidia lata {Sh, inches long; one specimen) possessed from 16 to 20, 

 and the species was defined upon the ground of this difference* and 

 of a peculiarity in the aperture of the dorsal tubercle. 



Take again the dorsal lamina. Heller unfortunately gives no 

 details upon this point, but Traustedt and Roule agree that the 

 lamina is strongly pectinated. In Roule^s specimens the right face 

 of the lamina is also provided with a few smaller " languettes.^' 

 On the other hand, Hancock's A. ruhicunda, Herdman's A. lata 

 (from Loch Long), and my specimens agree in being merely minutely 

 denticulated along the edge of the lamina. 



It is true, therefore, that we have at last arrived at a point 

 wherein some of the north Atlantic forms agree to differ from their 

 Mediterranean relatives ; but he would be rash who would distinguish 

 the species upon this ground alone, in view of the numerous cross- 

 resemblances in other respects. 



The prcebranchial zone is minutely tuberculated in my specimens 

 just as in Traustedt's. 



Altogether, therefore, there appears to be no sound reason why 

 the numerous mentuloid forms mentioned in this paper should not 

 be grouped together into one species and entitled Ascidia mentula. 

 Some other " species " might even be added to the list. Heller's 

 A. ruhescens has rightly been included by Roule as a young 

 individual of the species, and it is just possible that Herdman's 

 A. fusiformis (If inches long; three specimens) is merely a young 

 condition also. 



It is difficult to form an opinion upon Hancock's A. plana and 

 A. alderi ; but they appear to belong to this species also. 



I cannot hope to have altogether avoided error in the course of 

 this paper, but I have certainly endeavoui'ed to do so ; and I trust 

 that, as an attempt to throw a little light upon some of our British 

 Tuuicates, my essay will not be without useful results. 



Moreover, it would seem to be serviceable if a word or two 

 should be said upon the desirability of keeping in mind the facts of 

 variation, and of adopting some method by which the broad phe- 

 nomena of variability within the limits of a species can be properly 

 and systematically recorded. 



* Since the above was put in type, I have been enabled to examine some specimens of 

 A. mentula, which were dredged in Loch Long and are now under Mr. Hoyle's charge in 

 the Manchester Museum. The number of tentacles is so variable as to be only 18 in an 

 individual 4| inches long, while it is nearly 40 in an individual 3 inches long. 



