PEOF. W. A.. HEEDMAK ON BEITISH TUNICATA. 447 



draw upon a purer supply of water — and in any such ease the 

 advantage of having the entrance to the peribrauchial cavity 

 provided with a circlet of tentacles is obvious. 



I find tlie branchial sac m Polycarpa glomerata liable to very 

 considerable individual variation, and in figs. 11, 12, 13 on Plate 

 XXXV. I give the graphic branchial formula * of three indi- 

 viduals. From these it will be seen that the number of folds may 

 be four on each side, four on one side and three on the other, 

 or three on each side ; sometimes there are less than three folds. 

 Usually one fold (or more) on each side is rudimentary, i. e. is 

 really no longer a fold, and does not project into the cavity 

 of the sac, and in such cases it is only possible to recognize the 

 position of the missing or reduced fold by the approximation of 

 the internal longitudinal bars (see PI. XXXV. fig. 11 ; right 

 side IV., left side I.). 



The dorsal tubercle is crescentic, and lies obliquely in a shallow 

 peritubercular area (PL XXXV. fig. 9). 



POLTCAEPA QITADEANGULAEIS (Fories). (Pi. XXXVI. 



figs. 11, 12.) 



Cynthia quadrangularis, Fori., British Mollusca, vol. i. p. 38, 

 pi. D. fig. 1. 



This species was described by Porbes in 1853 from specimens 

 dredged by Mr. E. McAndrew and himself from a depth of 

 30 fathoms in Loch Pyne. So far 'as I am aware it has not 

 been recorded since, although I find a specimen of it, also from 

 Loch Pyne, amongst the Cynthiidae of Canon Norman's collec- 

 tion. I dredged in September 1892 a Polycarpa from a depth 

 of 80 fathoms, in Loch Pyne between Tarbert and Ardrishaig, 

 which corresponds so closely with Porbes's figure in the ' British 

 Mollusca' and with his short description that I am convinced 

 that it is the q^uadrangularis, and I am pleased to be able to 

 restore Porbes's species, and give the following sufficient anato- 

 mical description of it drawn up from an examination of a 

 specimen hailing from the original locality. 



A most marked feature in the external appearance is the pair 

 of long siphons, each of which is quadrangular in section and has 

 the large square aperture on its summit. The apertures fold 

 into an X shape in closing. 



* For the explanation of this brief method of expressing the condition of 

 the folds, bars, and stigmata of the branchial sac, see Herdman, " On individual 

 variation among Ascidians," Proc. Lit. and PhiL Soc. Liverpool, vol. xxxvi. 

 p. 313 (1882). 



