290 Messrs. Alder and Hancock on two new species of 



the following extract from the translation of his paper on the 

 Cystidea* will prove: — 



"Mr.. Austin states that Sycocrinites exhibits three dorso-central 

 plates," &c (see Annals of Nat. Hist vol. xi. p. 206). "This is 

 manifestly the description of a Crypt ocrinite (so named in 1840) ; 

 but this author does not state the locality of his specimen," &c. 



I will only add, that Cryptocrinus is a genus arranged with 

 Von Buch's family of Cystidea, and that it does not appear to 

 belong to a group along with Pentremites* 



August 9th, 1851. 



XXIII. — Descriptions of two new species of Nudihranchiate Mol- 

 lusca, one of them forming the type of a new Genus. By Joshua 

 Alder and Albany Hancock. With the Anatomy of t/ie 

 Genus, by Albany Hancock. 



[With two Plates.] 



The Nudibranchiate Mollusks, which we have now the pleasure 

 of introducing for the first time to the notice of naturalists, we 

 owe to the persevering researches of our friend Mr. W. P. Cocks 

 of Falmouth, by whom they have been communicated to us, with 

 kind permission to publish descriptions of them. The first spe- 

 cies we shall notice we refer to the Thecacera of Fleming, a genus 

 at present so imperfectly understood that any addition to our 

 knowledge of its characters may be considered as furnishing a 

 desideratum in this family of the Mollusca. We propose to 

 characterize it as follows : — 



Thecacera virescens. Body rather convex, smooth, of a light 

 peach-blossom tint, blotched with green anteriorly and poste- 

 riorly. Head with a plain sub velar margin in front. Ten- 

 tacles broadly laminated, the laminated portion green, the 

 lower or smooth portion of the same colour as the body ; they 

 are retractile within moderately-sized sheaths with smooth 

 margins. Branchial plumes five, green, margined with white. 

 A single row of obsolete tubercles encircles the branchial re- 

 gion. Foot of a dull yellowish white. Length T 3 n ths of an 

 inch. 



This beautiful little animal differs in several respects from the 

 Doris pennigera of Montagu, which is the type of the genus The- 

 cacera, and might by some naturalists be thought entitled to 

 rank as a new genus ; we prefer, however, to consider it an ab- 



* A translation of this paper appeared in the Journal of the Geological 

 Society, Feb. 1st, 1846. 



