CANADIAN FOSSILS. 25 



Yet notwithstanding this apparent passage established by such 

 forms as Eunema, it is probable that Loxonema belongs to an entirely- 

 different family (the Pijrumiddlidce), and that the rest are thin-shelled 

 forms of Litorinida, with an evident tendency towards the fragile 

 Ianthina, with which the rudimentary sinus in the outer lip also 

 helps to connect them. 



Professor Forbes thought Holopea (and therefore Cyclonema, which 

 is closely allied,) much like Litiopa, a pelagic form of the Litorinida, 

 and they both have the concave inner lip of Litorina. There is 

 some difficulty in determining the nearest recent analogue of 

 Trochonema. It may be compared with Skenea (Litorinida) or with 

 Adeorbis (i. e., Cyclostrema), which differs from the other Trochidse 

 in its non-nacreous shell. Eunema, it is true, has the sinus of the 

 outer lip so much increased as to separate it from any recent forms 

 of LitorinidcB, while it much resembles Ianthina or the allied genera 

 Recluzia, which has an an extremely simple, paludiniform shell. 

 It has a much thicker shell, however, than these, and its affinities 

 are with the fossil genera above described, from which the elongate 

 form and sinuous outer lip effectually distinguish it. 



The fossil shells under consideration appear to be all too solid for 

 comparison with floating shells, and the tendency they exhibit to 

 form projecting and irregular apertures in old age indicates rather a 

 ground-feeding and ultimately sedentary habit. 



Cyclonema. 



With regard to the limits between Holopea and Cyclonema, not 

 much that is satisfactory can be said. The form is similar, and the 

 striae or ribs are not always present in the one or absent in the other. 

 The possession of a concave reflected inner lip, if it could be estab- 

 lished for Cyclonema. would be a good character, yet certain species 

 in the Canadian collection which are quite smooth, and have the 

 general character of Holopea, resemble Cyclonema in this, though they 

 want the characteristic sculpture. Holopea seems to be sometimes 

 (H. obliqua, Hall,) umbilicate, and sometimes not so. Cyclonema is 

 never umbilicate, and the inner lip is concave in the type species. 



If accepted as a genus, Cyclonema should include all those Silurian 

 species hitherto referred to Turbo, Euomphalus, etc., which have 

 concentric ridges and oblique lines of growth. Although this is only 

 a superficial character, it is found in so many species, that, combined 

 with the thin shell, it may be taken into account. 



