Genus 3. TRIOPA * Johnston. 



Corpus limaci forme, subdepressum, postice acuminatum, pallio parvo appendicibus linearibus 

 marginato obtectum. Caput inferius, sub pallio absconditum, tentaculis 2 labialibus instructum. 

 Tentacula dorsalia 2, laminata, intra vaginas retractilia. Branchiae plumosse, in linea media dorsi anum 

 prsepositse. Apertura genitalis ad latus dextrum. 



i 



The type of this genus is the Doris clavigera of Miiller, a species long known to 

 naturalists by the figures and description of that author, though their familiarity with the 

 living niollusk is but of recent date. 



It was raised to the rank of a genus, under the above name, by Dr. Johnston in his 

 account of the ' Scottish Mollusca Nudibranchia,' published in the first volume of the 'Annals 

 of Natural History' in 1 838 ; but characterised rather loosely, in order, as the author informs 

 us, to embrace another species, which, however, has turned out to belong to the genus 

 Polycera. In redescribing it, we have, therefore, found it necessary to make a considerable 

 alteration in the original generic character. In the first volume of the ' Enumeratio Mollus- 

 corum Siciliae/ published in 1836, Dr. Philippi has described a genus under the name of 

 TJuplocamus, founded upon a Mediterranean species, differing only from this in the branched 

 nature of the marginal appendages of the cloak, which he considers to be additional branchise, 

 and in the absence of distinct oral tentacles. The genera are probably distinct; but however 

 that may be, the name of Triopa must be retained, as that of JEuplocamus had been previously 

 used for a genus of insects. M. Philippi has since united his genus with the Idalia of 

 Leuckart, a proceeding in the propriety of which we do not coincide. 



The only two species of Triopa at present known are inhabitants of the seas of Northern 

 Europe. Two species of Euplocamus are found in the Mediterranean. Yellow appears to be 

 the predominant colour in all of them. 



The form of the body, and the marginal processes of the cloak, in Triopa, indicate an 

 approach to the Tritoniadce, which is further carried out by these processes assuming a 

 branched form in Euplocamus, having a considerable resemblance to the branchial tufts in 

 Tritonia. And, assuming that the lateral processes are of a branchial nature, Triopa may be 

 considered to make an approach to that section of the JEolididce, which have a posterior dorsal 

 anus (the Proctonotina), for in them the branchial papillae pass round the front of the head 

 in the same manner as the processes do in this genus. The head and oral tentacles have 

 also some similarity to those of Antiopa. In its well defined cloak, rough with spicula and 

 tubercles, Triopa shows, on the other hand, a near affinity to Boris. We have been chiefly 

 influenced by the character of the lingual plates, and the disposition of the branchial plumes, 

 in giving it a place amongst the Tolycerince. 



* The name of a son of Neptune. 



