10 CANADIAN FOSSILS. 



upper or convex side is constantly overgrown with sponge (Stromato- 

 cerium rugosum*), while the flat or lower side preserves the sharp lines 

 of growth, which would have been abraded had the animal been 

 endowed with much locomotion. 



Kaphistoma, Helicotoma, and Ophileta. 



Decade I. Plates II. and III. 



Genus Scalites, Conrad. Mollusca Gasteropoda. Family Ianthinidse. 

 Shell thin, turbinate or depressed, with angular whorls, flat 

 above ; aperture deeply notched, but without a band. 



Subgenus Scalites. Turbinate ; whorls flat above, turrited, pro- 

 duced below ; umbilicus none. 'Form elongate. 



Sub-genus Rapkistoma, Hall. Depressed, often discoid ; spire flat, 

 or only gently convex, with close sutures; whorls acute angu- 

 lar externally, and often with an angular edge to the moderate 

 umbilicus. Form lenticular. 



Sub-genus Helicotoma, Salter. Depressed, discoid ; spire nearly flat ; 

 whorls obtusely angular externally, rounded below ; umbilicus 

 broad. Form cirrhoid or helicoid. 



Sub-genus Ophileta, Vanuxem. Discoidal ; spire sunk above ; 

 umbilicus below perfectly open, and exposing the whorls all on 

 one plane; whorls numerous, truncate and biangular exteriorly; 

 mouth trigonal. Forms with deeply concave spires. 



On mature consideration I cannot find any reason for separating the 

 above series of forms, except as sub-genera. They pass into each 

 other by almost insensible degrees, some species of Raphistoma, for 

 instance, being merely depressed Scalites, while others need but a little 

 more angularity below to become species of Ophileta. Helicotoma 

 is a new sub-generic form, which I have been obliged to institute in 

 order to express a middle term of the series, one in which the true 

 discoid form of Raphistoma is maintained, without the extreme angu- 

 larity of that genu?, yet with a spire almost as much sunk and an 



* A fossil of the Chazy and Birdseye limestones of New York. The Maclurea may 

 thus be regarded as representing in this rich locality the Chazy limestone, which is still 

 farther illustrated by such species as Bellerophon sulcatinus and B. rotundaius. See 

 Preface. 



