REPORT ON THE TUNIC ATA OF PLYMOUTH. 63 



In 1853 Professors Forbes and Goodsir announced the discovery 

 of a composite Tunicate allied to Savigny's Diazona violacea, but 

 differing from it in the possession of the following characters : 

 Plain undivided orifices, non-pedunculated abdomen, meshes with 

 " one ciliated opening " only, and apple-green colour. Their genus 

 Syntethys was established upon these grounds, but Alder subse- 

 quently wrote to show the generic identity of the two forms, basing his 

 criticisms upon an examination of specimens dredged near Guernsey 

 and possibly upon a re-examination of a portion of one of the 

 original specimens of Forbes and Goodsir. Alder satisfactorily 

 showed that the difference of colour was one due entirely to the 

 action of the spirit used in preservation, and also that the peduncu- 

 lation of the abdomen is very variable in its extent. He also noted 

 that, after preservation, the division of the apertures into lobes was 

 generally difiicult to make out. His conclusion in regard to the 

 generic identity of Diazona violacea and the so-called Syntethys is 

 probably correct, although, as I shall endeavour to show below, his 

 identification of the Guernsey species with that of Forbes and 

 Goodsir from the Hebrides is extremely doubtful. 



4. Diazona violacea, Savigny. (PI, II, figs. 7, 8.) 



Diazona violacea, Savigny. Memoh-es, pp. 35 — 38, 175, 176, pi. xii. 



— — Fleming. Moll. Animals, 1837, p. 211. 



— mediteeeanea, Dujardtn. L. c, pp. 499, 500. 



— HEBEIDICA, Alder. L. c, p. 169. 



— VIOLACEA, Cams. L. c, pp. 480, 481. 



Colony massive, irregularly rounded, attached by a short, thick 

 pedicle or base ; total diameter about 7 inches, total height 5 or 6 

 inches ; of apple-green colour when alive, semi-transparent. 



Zooids often 2 inches long, with oral and cloacal orifices each 

 six-rayed. 



Branchial sac with sixty to eighty transverse rows of stigmata ; 

 meshes each containing three, rarely four stigmata ; internal longi- 

 tudinal bars for the most part completely formed, but here and 

 there represented by T-shaped interserial papillge, as in Tylo- 

 hranchion ; dorsal tubercle a large deep groove, elongate antero- 

 posteriorly, with thickened walls. 



Habits. — Attached to rocks or stones in deep water. 



Dredged at Plymouth on rough ground off Stoke Point, and off 

 the Eddy stone in 20 — 40 fathoms of water. 



There are two remarkable statements in the original description 

 of the structure of Syntethys Hehridicus by Forbes and Goodsir 



