Dr. Johnston on Scottish Mollusca. 49 



while in the ovum, has been observed to have a rotatory mo- 

 tion produced by vibratile cilia placed on the exterior ; a mo- 

 tion, which appears destined to bring a constant supply and 

 renewal of sea-water into the interior of the organization, in 

 order to perfect the animal before it is, as it were, launched 

 into the ocean. Possibly, also, the continued friction of the 

 cilia against the interior of the egg may tend to abrade it, and 

 open a passage for the young animal*. 



There are few Scottish species of the order hitherto ascer- 

 tained, but I have found the task of determining them, and 

 their synonymy, very difficult and irksome. This has resulted 

 partly from the meagreness of their descriptions in Linnasan 

 authors, partly from the want of access to some expensive 

 works containing figures that might have supplied this de- 

 ficiency of detail, and partly from the difficulty of ascertaining 

 the extent of variation in the species themselves. In the hope, 

 however, of drawing attention to so interesting a tribe, and of 

 laying a foundation for a more accurate knowledge of it, I offer 

 the result of my study to the British naturalist, persuaded that 

 he will receive with indulgence an essay that neither care nor 

 industry has been able to free from imperfection and doubts. 



Family I. DORID^S. 

 Branchiae retractile, posterior and dorsal, placed in a circle 

 round the anus. Cyclobranchiata, Blainville, Man. 488. 

 — Urobranchia, Latreille, Fam. du Reg. Anim. 173. — Les 

 Doris, Rang, Man. 131. 



1. Doris. Dorsal tentacula lamellated : anus without scales. 



Family II. TRITONIAD^E. 



Branchiae non-retractile, lateral and dorsal : anus lateral and 

 anterior. Polybranchiata, Blainville, Man. 484. 



* BranchicE arbuscular : dorsal tentacula two, retractile within a sheath at 

 their base. Dicerata, Blainville, Man. 487- — Seribranchia, La- 

 treille, Fam. da Beg. Anim. 174. — Les Tritonies, Bang, Man. 128. 



2. Tritonia. Branchiae arborescent: tentacula fissile and 

 pectinate. 



3. Melibea. Branchiae ovate, muricate : tentacula filiform^ 

 simple. 



* Roget, Bridgew. Treatise, i. 216. 

 Ann. Nat. Hist. Vol. 1.— No. 1. March 1838. e 



