Dr. Johnston on Scottish Mollusca. 115 



tapered behind, plane. — The structure of the tongue is wonder- 

 fully fine, and forms an excellent object for the microscope. 



Tritonia Hombergii of Blainville, Man. de Malacologie, 

 p. 487. ph 46. fig. 6. is either distinct or very badly drawn. 



2. T. arborescens, branchial tufts distinct, five or six on each 

 side ; oral veil in four arborescent lobes. 



Tritonia arborescens, Cuv. Mem. vi. 28. pi. 1. fig. 8 — 10. Fleming in 

 Edin. Encyclop. xiv. 619. Lam. Anim. s. Vert. vi. i. 304. Edin. Phil, 

 Journ. ix. 254. Grant in ibid. xiv. 165 and 185. Flem. Brit. Anim. 

 284. Stark, Elem. ii. 68. — Doris arborescens, Mull. Zool. Dan. Prod. 

 229. Fair. Faun. Grcenl. 346. Turt. Gmel. iv. 79. Bosc. Vers. i. 

 112. — Doris cervina? Turt. Gmel. iv. 78. — Tritonia cervina? Bosc. 

 Vers. i. 106. 



Hab. Frith of Forth, Dr. Grant. Loch Broom, Ross-shire ; and Zetland 

 Isles, Rev. Dr. Fleming. 



Desc. "Length about an inch ; foot narrow, sides com- 

 pressed ; cloak smooth, its margin above the mouth with four 

 plumose appendages ; branchice decreasing in size towards the 

 tail ; tentacula conical, transversely striated ; the sheath with 

 a divided margin. — I have found this species in the Zetland 

 Isles, agreeing with the characters of Cuvier, with this differ- 

 ence ; that the branchiae in his are only five on each side, 

 while in our specimen there appeared to be six. But as the 

 two posterior ones are very small, and as his examples were 

 preserved in spirits, it is probable that they have escaped de- 

 tection." Rev, Dr. Fleming, — Dr. Grant has noticed this spe- 

 cies to emit at intervals a peculiar and very audible sound. 

 " The sounds," he says, " obviously proceed from the mouth 

 of the animal ; and at the instant of the stroke we observe the 

 lips suddenly separate, as if to allow the water to rush into a 

 small vacuum formed within. As these animals are herma- 

 phrodites, requiring mutual impregnation, the sounds may pos- 

 sibly be a means of communication between them ; or if they 

 be of an electric nature, they may be a means of defending 

 from foreign enemies, one of the most delicate, defenceless, and 

 beautiful gasteropods that inhabit the deep." 



3. T.plebeia, branchial tufts distinct, five or six on each 

 side ; oral veil entire, sinuate in the middle, the margin ten- 

 tacular. Plate III. fig. 3—4. 



Tritonia plebeia, Johnston in Edin. New Phil. Journ. v. 77. 



I 2 



