ELIOT : ON DORIS PL AN AT A. 181 



other distinctly trifid.^ This divisibility of the posterior plume no 

 doubt explains the apparent variations in number. The largest flat 

 grey specimen has seven, of which two might also be considered as 

 a single but divided plume. In the other two specimens eight tips 

 are visible, the plumes being retracted. In the larger white specimen 

 there are six or eight plumes, according as the posterior one on each 

 side is counted as bifid or as two ; in the smaller example, there 

 appear to be nine, quite separate. In both white specimens the 

 branchial apparatus is entirely everted, and the anal papilla unusually 

 large. In all specimens the plumes are small and scanty, apparently 

 tripinnate. 



The buccal mass is greenish, small, and contains a labial armature 

 of short, closely packed brown rods, arranged in an almost complete 

 ring. The radula is fragile and not large. There are about 1 8 rows 

 of colourless teeth, and the formula varies from 9 + 12-0-12 + 9 to 

 10 + 14-0-14 + 10. I could not find any row which was wider than 

 this, but such may have existed and been broken up. The 12 or 14 

 teeth nearest the rhachis are of the ordinary hamate type, and strongly 

 built. The 9 or 10 outermost are extremely thin, and closely crowded 

 together. Bergh's plates (I.e.) give a good idea of both kinds of teeth. 

 The other internal organs appear to be as in the genus Geitodoris, 

 but in one of the white specimens the seminal duct and glans penis 

 appear to bear minute hexagonal scales. 



These forms cannot be referred to Platydoris, for not only do they 

 differ decisively in the mouth parts, but they have not the characteristic 

 stiff, leathery consistency and feeling. On the other hand, they have 

 all the essential characters of Geitodoris, and Venill, who discovered 

 Geitodoris complanata, thought it might be allied to D. planata, A. & H. 



I regard the form here examined as being certainly identical with 

 D. planata, and as belonging to the genus Geitodoris. The only 

 question is whether it should be specifically distinguished from 

 Geitodoris complanata found on the north-east coast of America. The 

 colour of the two is similar, and the chief differences seem to be that 

 the specimens from Plymouth are (1) smaller, (2) have a smaller 

 radula, (3) have varying branchise, which appear to be typically six, 

 with a tendency to division in the posterior plumes, whereas in 

 G. complanata there are definitely ten plumes. All these differences 

 could be explained by the hypothesis of growth, but further exami- 

 nation may prove that there are two species, or well-marked varieties, 

 from the east and west coasts of the Northern Atlantic respectively. 

 The specitic name planata (A. & H., 1855) has clearly priority, and 

 must be borne by the American form unless it is shown to be distinct. 



In view of Garstang's description of the buccal parts of his Platydoris 

 planata it is possible that it may be really distinct from the specimens 

 sent me. Both Verrill's G. complanata and the form here examined 

 were obtained by dredging, and seem to frequent fairly deep water. 



' He also adds that the branchiae of each side are retractile separately from those of 

 the other side. 



