CARBONIFEROUS AGE. 45 



more or less deep marginal slit or sinus near the middle, at the termina- 

 tion of a revolving band usually seen on all the whorls ; sinus sometimes 

 closed, excepting at intervals, so as to leave a series of isolated openings 

 in the revolving band. 



The animal of the typical genus (Pleuroiomai-ia) of this extensive and interest- 

 ing family, now so nearly extinct, has not, we believe, been described. That of 

 /SfeisswreZZa, however, an existing genus apparently related to this family (though 

 its shell is not pearly within), has been studied by Mr. Lucas Barrett, who describes 

 it as follows : " Tentacles long, serrate, at the base of which are placed the eyes ; 

 foot furnished with two pointed lappets, and two long, slender, serrated cirrhi on 

 each side. Operculum very thin, ovate, with an obsciu-e, subspiral nucleus. No 

 part of the animal was external to the sheU. The only living specimen occurred 

 at Hammerfest, ia forty-six fathoms of water." 



The family Pleurotomariidce was represented during the Palaeozoic and several 

 later epochs, by a great number of beautiful shells, presenting elegantly sculptured 

 surfaces. It seems to stand, as it were, between the Trochidce and the Haliotidce, 

 though authors are not agreed in regard to its relations to these and some of the 

 allied groups. It includes the genera Pleurotomaria, Platyschisma, Scissurella? 

 Murchisonia, Troclioioma, and Polytremaria. The Palaeozoic groups Euomplialus' 

 (as typified by such forms as E. pentangulatus, Sowerby), Scalites, BapJiistoma, 

 , Helicotoma, and some undescribed genera from the older rocks, seem also to be 

 related to this family. 



Genus PLEUROTOMAEIA, Defrance. 



Synon. — Anatomus [??J, Montport, Conch. Syst. II, 1810, p. 278. 



PUurotomaria, Defrance, Diet. Sci. Nat. XLI, 1826, 381. — Menke, Synon. 1828; and ib. 1830, 55. — Db- 

 SHATES, Encyc. Meth. Ill, tab. 1830 ; and ib. 1832 ; 789. 

 Etijm. — Tt\evf!t, side ; te^vso, to cut. 

 Examp. — PUurotomaria Quoyana, FisCHEK. 



Shell trochiform, or more or less- conical, pearly within, and variable in thickness 

 according to the species, with or without an umbilicus; volutions angular, flat- 

 tened or rounded. Surface variously ornamented with striae, nodes, granulations 

 or carinae. Aperture subquadrate, semi-oval, suborbicidar or subrhombic; inner 

 lip usuaUy thin ; fissure of outer hp generally narrow and deep ; revolving band 

 corresponding in breadth with the sinus. 



The shells included in this genus are very similar in form, and the possession of 

 a fissured lip, to those of the recent genus Scissurella, but differ in size and tex- 

 ture — all the kno'wn species of Scissurella being minute, non-perlaceous shells. 

 The Pleurotomarias also closely resemble the genus Anatonms, of Montfort, from 

 which Hermannsen and some others think they are not distinct. Other authors 

 regard Scissurella, D'Orbigny, and Anatomus, of Montfort, as synonymous. Judg- 

 ing from Montfort' s description of the genus Anatomus, however, it seems scarcely 

 possible that it can be identical with Scissurella, since he distinctly states that 

 the typical species of his genus is a pearly shell, and that the animal is without 



