ESCH— PTEROPODA AND HETEROPODA 175 



years later Cuthbert Collingwood*, who collected some specimens of Pterosoma in the 

 channel of Formosa and ascribed them to the same species as that of Lesson, should never- 

 theless have been somewhat doubtful about his determination. He did not however give 

 any further information about his specimens, only stating that they belonged in fact to 

 the Heteropods. 



Under these circumstances it is easy to understand how Moseley fell into the error, on 

 the suggestion of von Willemoes Suhm, of identifying Lesson's Pterosoma with a pelagic 

 Nemertean that he was describing"!". Moseley's opinion has been adopted by Fischerj. 

 It was not before the year 1895, when Whitelegge was fortunate enough to collect some 

 specimens, cast ashore by an easterly gale at Mavoubi-a Bay near Sydney, and handed over 

 his specimens to Hedley, that the doubts about the mysterious genus Pterosoma were 

 definitely removed. 



Hedley § showed the true nature of Pterosoma, described the general characters of the 

 body, fin, proboscis and shell from the only intact specimen at his disposal, and gave the 

 first figure of the radula, thus showing its affinity to the Heteropods in general. He did 

 not express his conviction that his species and that of Lesson, Pt. plana (strictly speaking, 

 planum, as has been rightly advanced by Crosse||) were identical, though this seems 

 to result from his description. 



I myself II lately recorded Pterosoma among the material of the Challenger Expedition 

 and felt justified in bestowing a new name, Pt. challengeri, upon it. After further study 

 of Hedley 's figures I think, however, that no true specific difference exists, and it is for 

 this reason that I readily give up my new species, being now convinced that it is the same 

 as Pt. planum. 



I now describe the species more accurately than former circumstances permitted me. 



11. Pterosoma planum (Lesson). (Plate 13, figs. 1 — 5.) 



Stat, kk (1), Desroches Atoll (Amirante Group). Only one specimen ($) was 

 caught, fortunately without any injury, except to the shell. 



As previous authors have mentioned, the animal is remarkable for its broad, wing-like 

 expansions at either side of the body. These are produced by the strong development of 

 the cutis, which is very thick on the body and beyond this forms the large wings, gradually 

 thinning out at the margins**. The thickening of the cutis begins just at the transition 

 between proboscis and body, before the eyes ; distally from the visceral nucleus it becomes 

 thinner, finally almost disappearing on the tail (fig. 2). Seen from above (fig. 1) the 

 animal is broadly heart-shaped, with a small emargination between the eyes, ending 



* Rambles of a Naturalist, 1868, p. 54. 



t On a young specimen of Pelagonemertes rollestoni, Ann. May. Nat. Hist. (4), vol. xvi. pp. 37" — 382, 

 PI. xi. 1875. 



% Manuel de Conchy lioloyie, p. 537, 1883 — 1887. 



§ Pterosoma Lesson claimed as a Heteropod, Proc. Malae. Sue. Loudon, vol. i. 1895, pp. 333 — 335, 

 figs. 1 — 4. The author was apparently not acquainted with Moseley's hypothesis, else he would not have 

 indicated Fischer's opinion as "a mere conjecture." 



|| Note sur le genre Pterosoma Lesson, Journ. de Conchylioloyie, vol. xliv. pp. 207 — 212, 189G. 



II I.e. pp. 22, 23, PI. in. fig. 75. 



** According to Hedley (I.e. p. 334) the wings are not thinner at the margins. 



