ONISCIGASTEE WAKEFIELDI. 145 



But in this species there is an enormous development of the 

 thoracic surface, this portion of the body forming a kind of 

 carapace, covering all but the terminal segments of the abdomen, 

 and concealing the rudiments of wings. And it must be noted 

 that the formation of the abdominal segments in the aquatic con- 

 ditions of Bfstisca disappear in the perfect and subimaginal con- 

 ditions, which are only remarkable for the very obese thorax. 

 The formation is again seen in the terminal segments of the 

 extraordinary animals described by Latreille as a genus of 

 branchiopod Crustacea under the name oi I'rosopistoma, but which, 

 I think, are now sufficiently proved by the French entomologists 

 N, and E. Joly (father and son) to be the aquatic conditions of 

 some unknown species of Ephemeridse, although when they first 

 expressed the idea of such a connexion I confess to having been 

 sceptical. The typical examples of Prosopistoma may be re- 

 garded as belonging to an insect inhabiting Madagascar, and are 

 now in the Hopeian collection at Oxford, in charge of Professor 

 AVestwood. But the form also occurs in France, and was figured 

 and described by Greofiroy in the ' Histoire abregee des Insectes 

 de Paris ' under the name of ' Binocle a queue en plumet.' It 

 was re-found many years afterwards by Dumeril in the Bois de 

 Boulogne, but again disappeared until the Messieurs Joly found 

 it in the Garonne, at Toulouse. It, like Bcetisca, has also an 

 enormous carapace, but of a more rounded form. Latreille de- 

 scribed it as Prosopistoma punctifrons, placing it, as before stated, 

 among the branchiopod Crustacea ; and though succeeding authors 

 copied his description, its position among the Crustacea was 

 never thoroughly admitted. I think we must accord to the Jolys 

 the merit of having discovered its true position : but it is hard to 

 imagine what the perfect insect can be like ; for no European 

 species yet known shows any approach towards the characters so 

 prominent in these aquatic creatures *. That these are not 

 Crustaceous is sufficiently proved by the fact that the Messieurs 

 Joly have at length discovered five pairs of gills on the first five 



* One is tempted to ask the questions: — Can there be minute apterous 

 Ephemerides ? and can the imago of Prosopistoma be in that condition ? Such 

 a thing is by no means impossible ; for apterous exceptions exist in almost all 

 groups of winged insects. If so, it would account for the absence of wing- 

 rudiments in all the individuals dissected by the Jolys. The solution of the 

 mystery surrounding Prosopistoma is waited for impatiently by all entomolo- 

 logists who take an interest in the more philosophical branch of the science. 



