Genus 5. THECACERA,* Fleming, 



Corpus liniaciforme, lseve, non palliatum, postice acuminatum. Caput subinferius, velo aut piano 

 aut lobato sepissime obtectum. Tentacula 2 dorsalia, laminata, intra vaginas retractilia. Branchice 

 plumosse, in linea media dorsi anum circumdantes, appendicibus lateralibus linearibus vel tubercularibus. 

 Apertura genitalis ad latus dextrum. 



Thecacera is very nearly allied to Polycera, from which it chiefly differs in having sheathed 

 tentacles ; the laminated character of these organs, together with the smooth surface of the 

 body, sufficiently distinguish it from JEgirus ; and with other genera of the group it cannot 

 readily be confounded. The genus was proposed by Dr. Fleming in his ' History of British 

 Animals' (1828) for the Doris pennigera of Montagu, a rare and curious Mollusk, then only 

 known by the description of the latter author, and even up to the present time involved in 

 much obscurity; but possessing characters, which, Dr. Fleming very properly observed, 

 "mark it as the type of a genus." Another species has lately been discovered by Mr. Cocks 

 at Falmouth ; and more recently still, a third species has been dredged off St. Ives by 

 Mr. Barlee. These agree with that of Montagu, in the smooth body, and laminated and 

 sheathed tentacles, but vary in almost every other respect from the typical form. 



The only known foreign species of this genus was found by Mr. Charles Darwin in the 

 Chomos Archipelago, south of Chiloe, and is not yet published. 



Thecacera has the body limaciform and smooth, swelling a little in the centre, and 

 terminating in a pointed tail. The head is sub-inferior and furnished, like Polycera, with 

 corneous jaws : it is generally covered by a velar expansion, more or less distinct, and of 

 variable character, being either even, lobed, or tuberculated. The tentacles are two in 

 number, dorsal, and laminated ; retractile within even or foliated sheaths. There is no dorsal 

 area or rudimentary cloak. The branchiae are plumose, surrounding the vent in the centre 

 of the back posteriorly. They are furnished with one or more appendages on each side, 

 which in some species are large and well developed, in others tubercular and obscure. The foot 

 is of the usual form, and co-extensive with the body. The aperture of the sexual organs is 

 on the right side. 



The anatomy of 'Thecacera appears to be very similar to that of Polycera ; but on account 

 of the rareness of the genus, we have not been able to examine it in detail. We have 

 ascertained, however, some particulars respecting the buccal apparatus and the nervous 

 system. 



The buccal organ and jaws of T. virescens closely resemble those of Polycera Lessonii, 

 the jaws having very imperfect cutting edges : in T. capitata the jaws are, however, more like 

 those of P. quadrilineata. The tongue of the former species has eighteen or nineteen rows of 



* From Orjici], a sheath, and icipag, a horn. 



