224 EOCENE MOLLUSCA. 
assuming the sub-conical form described by Solander as the chief distinction between 
P. exorta and P. macilenta. The outer lip is thin and much arched; and the sinus, 
which is wide and sub-trigonal, is placed in the very middle of the margin. 
The elevated transverse line on the shoulders of the whorls, broken by the 
undulations caused by the ribs, occasionally presents a resemblance to the denticula- 
tion characteristic of P. dentata; and this circumstance probably led to P. exorta 
being considered as identical with P. dentata. The two species are, however, 
distinct; in this the shell is wider and shorter, the spire thicker and more conical, 
the whorls not so much angulated, the posterior margins narrower and less depressed, 
and the anterior canal shorter and more conical. 
Nyst,in his ‘Recherches sur les coquilles fossiles de la province d’Anvers,’ has recorded 
certain shells from Boom which he has referred to the present species, and for which he 
has given Sowerby’s diagnosis verbatim. Subsequently, De Koninck, in his ‘ Coquilles 
fossiles de Basele, Boom, &c.,’ described some shells from those localities under the name 
P. regularis, and other shells he referred to the present species. These different shells, 
Nyst, in his later work, ‘ Description des coquilles, &c., dela Belgique,’ considered to be 
merely individuals of the same species in different stages of growth, and that author 
referred them, erroneously as I have already shown, to P. rostrata (Sol.) Unfortunately 
I have not any specimens of the Belgian species to refer to; but, assuming that 
De Koninck’s P. exorta is but the young shell of his P. regularis, and judging from 
the figures given by him of the latter species and from those given by Nyst of the so- 
called P. rostrata, it is impossible to accept the identification of the Belgian shells with 
the present species as correct; a conclusion at which D’Orbigny has apparently also 
arrived, as that author has placed P. rostrata (Nyst) and P. regularis (De Kon.) in his 
“26th étage,” under the specific name P. exorta (Nyst), in forgetfulness, probably, of 
that name having been previously used by Solander. 
Size.—Axis, | inch and 1-12th; diameter, 5-12ths of an inch. 
Localities —Barton, Alum Bay (No. 29, Prestwich), Highcliff, Bracklesham Bay. 
No. 149. PLEUROTOMA MACILENTA, Solander. Tab. XXVI, fig. 13 a, 6. 
Murex Maciientvs, Sol. 1766. Brand., Foss. Hanton., p. 30, fig. 33. 
P. testa elongato-fusiformi, angustd, turritd, undique spiraliter lineata, longitudinaliter 
costatd: spira elevatd, acuminata : anfractibus convexiusculis, angulatis ; marginibus posticis 
concavis, ad suturam paulum incrassatis, transversim subtilissime lineatis ; lineis spiralibus 
numerosis, inequalibus, quibusdam filiformibus pretenuibus, inter alias eminentiores, acutas 
apparentibus ; costis numerosis, angustis, obliquis, fere ad caudam tendentibus: aperturd 
