On Pterochilus, and two new species of Doris. 329 



though rarely, obovate, very tender and delicate, easily torn, and 

 sometimes perforated with a few roundish apertures, undulated, 

 but not crisped. Frustules single or binate, scattered in the ge- 

 latinous substance of the frond without order, oblong, variable 

 in breadth, with the extremities rounded ; when fresh marked, ac- 

 cording to Dr. Dickie, at the four angles with a round colourless 

 spot. When dry the colouring matter contracts on either side 

 and the pale spots are not visible. There is no trace of any striae. 

 The lateral view is very narrow, simply oblong, with the ends 

 rounded. The colour of the plant when dry is a pale green. The 

 frustules do not appear to be decidedly siliceous, but they have 

 the prismatic form of really siliceous frustules. It appears to be 

 quite a spring species. Every specimen had vanished by the end 

 of the month. 



Plate IX. fig. 1. a, Dic/cieia ulvoides, nat. size, in different stages of 

 growth ; b, frustules sketched when fresh by Dr. Dickie, highly magnified ; 

 c, a frustule observed when dry ; d, lateral view of the same ; e, a portion of 

 the frond, less highly magnified, showing the simple and binate frustules. 



XLII. — Descriptions 0/ Pterochilus, a new genus of Nudibranchiate 

 Mollusca, and two new species of Doris. By Joshua Alder 

 and Albany Hancock, Esqrs. 



Gen. Pterochilus. 



Body oblong, nearly linear, tapering behind. Head anterior, ter- 

 minal, having a flat expanded lobe on each side, forming a kind 

 of veil above the mouth. Jaws corneous. Tentacula two, linear, 

 dorsal, with the eyes behind them. Branchiae papillary, elon- 

 gated, arranged down the sides of the back. Anus on the right 

 side behind the generative organs. 



This genus is allied to Eolis and Calliopcsa, but differs from 

 both in having the head strongly lobed at the sides. It may also 

 be distinguished from the former by having only two tentacula, 

 and from the latter by the gastric system, which in Calliopcea has 

 two longitudinal vessels down the back, while in Pterochilus there 

 is only one, which is central and undulating. The jaws are tri- 

 angular horny plates, capped at the anterior angles as in Procto- 

 notus. The tongue is narrow, strap-shaped and denticulated. The 

 auditory capsule has a single otolite. 



P. pulcher. — Body nearly linear, pale flesh-coloured, spotted 

 with opake white. Head furnished with a flat, rounded lobe on 

 each side of the mouth, forming a kind of veil. Tentacula short, 

 cylindrical, set much apart on the head. Eyes considerably be- 

 hind them. Branchiae five or six on each side of the back, in a 

 single series, the first two nearly opposite each other ; the rest 



Ann, §• Mag. N. Hist. Vol. xiv, Z 



