108 MARINE FAUNA OF ST. ANDREWS. 



Fam. ;5. Mesostomea, 0. Schm. 

 Genus MESOSTOMUM (Duges),M. Sch. 



Mesostomum bifidum, n. sp. 

 [Plate VIII. figs. 3-6.] 

 On the under surface of stones from low-water mark, East 

 liocks. One tenth of an inch long, and of a very pretty pale 

 orange hue. The body is pointed anteriorly, dilates in the 

 middle, and diminishes posteriorly, terminating in two pro- 

 cesses which have a few rather large papilla; on their crenated 

 edge ; these papillae seem to have a slight sucker-action. 

 There are two semilunar eyes, with the concavity external. 

 The cilia are specially distinct a little behind the snout, on 

 each side, at points corresponding to the long ciliary whips of 

 the developing Nemertean. The male organ formed a spirally 

 marked conical process behind the large median sucker; and 

 the testes were loaded with spermatozoa in various stages of 

 development. 



Fam. 4. Derostomea, OZrst. 

 Genus Voktex, Ehrenberg. 



Vortex capitate, ffirst. Entwnrf Plattwiinner, p. 65, 

 ))1. 1. f. 7. 



[Plate VIII. figs. 7-10.] 

 Occasionally under stones between tide-marks. Many dia- 

 toms occur in the digestive canal. 



Fam. 5. Opistomea, O. Schm. 

 Genus MONOCELIS, Ehrb. 

 Honocelis unipunctata (Fab.), OZrst. Ent. Plattw. p. 56. 

 This appears to be the Planar ia flustrm of Dalyell. It is 

 abundant under stones between tide-marks. 



Monocelis rutilanSjO. F. Miiller, Zool. Danie. iii. p. 49, 

 tab. Ill'), f. 10&11. 



Occasionally in the laminarian region. 



