HYDROIDA. BALE. 307 



The only species of which the generic position may be a 

 matter of doubt are those of the last group. In Nutting's 

 classification the corbula-bearing species are divided into 

 two genera Aglaophenia, comprising those forms in which the 

 corbula is closed, composed of broad leaflets, and unprovided 

 with hydrothecae ; and Thecocarpus, in which the corbula 

 is open, composed of narrow rod- or sabre-shaped ribs, with 

 a hydrotheca at the base of each rib. The species at present 

 under consideration form an intermediate group, having the 

 closed broad-leaved corbula characteristic of the restricted 

 genus Aglaophenia, but bearing hydrothecae as in Thecocarpus. 

 In 1907, however, Billard described a species, T. giardi, 1 

 with similar corbula?, and referred it to Thecocarpus, remarking 

 that previously only species were known in which the ribs 

 were free and the corbula open. 2 



According to Billard, then, the presence or absence of the 

 hydrothecse at the bases of the ribs is the sole criterion for 

 dividing the species, and even this is not constant, as the 

 same observer has described species in which the hydrothecae 

 are present in the female corbulae only ; while in the form 

 which I have described under the name of A. carinata, and 

 which Billard finds identical with Plumularia brachiata, 

 Lamarck, the receptacles at the bases of the leaflets, which 

 Billard considers hydrothecse, but which seem to me to be 

 sarcothecae, are often replaced by two or several undoubted 

 sarcothecae, the structure, so far as those particular leaflets 

 are concerned, being then the same as in the crucialis group. 



Stechow admits the genus Thecocarpus in the same sense 

 as Nutting, dividing the corbula-bearing species into three 

 groups (1) species with open corbulae and a hydrotheca at 

 the base of each rib = Thecocarpus, Nutting ; (2) species with 

 open corbulae and no hydro thecae=yl. whiteleggei, and other 

 species constituting the divaricata group ; (3) species with 

 closed corbulae and no hydro thecae^^gr/ao/j^ema. This 

 grouping, however, is obviously incomplete, no provision 

 being made for the fourth subdivision, namely, for the species 

 with closed corbulae but with hydrothecae, such as those 

 before us. 



Evidently if the genus Thecocarpus is to be accepted at all 

 it must be in the sense in which Billard adopted it, and 

 Nutting's diagnosis should be modified accordingly. 



1. Billard -Arch, cte Zool. exp. ct gen., (4), vi., Notes et Revue, p. 

 Ixxix. 



2. Thin was an oversight, as I had in 1881 described .-(. heterocarpa 

 (now known to be identical with Plumularia brci'irostris, Busk), in which 

 the corbula is similar to that of A. pltnna, but with a well-developed 

 hydrotheca on each leaflet. 



