•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' An vers,' 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, de la 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 etage," under the specific name P. exorta (Nyst), in forgetfulness, probably, of 

 that name having been previously used by Solander. 



Size. — Axis, 1 inch and 1-1 2th ; 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, b. 



Murex macilentus, Sol. 1/66. Brand., Foss. Hauton., p. 30, fig. 33. 



P. testa elongato-fusiformi, angustd, turritd, undique spiraliter lineatd, longitudinaliter 

 costatd: spird elevatd, acuminata : anfractibus convexiusculis, angulatis ; marginibus posticus 

 concavis, ad suturam paulum incrassatis, transversim subtilissime lineatis ; lineis spiralibus 

 numerosis, incequalibus, quibusdam filiformibus pratenuibus, inter alias eminentiores, acutas 

 apparentibw ; costis numerosis, angustis, obliquis, fere ad caudam tendentibus : aperturd 



