296 MRS. JANE LOXGSTAFF OX SOME NEW [Sept. I9I2, 



that I have seen have 011I3' the flange preserved ; and, when this 

 is broken, the interior is crystalline aud no further structure is 

 perceptible. In one of these the centre is darker, and suggests the 

 primary existence of a cavity which has been gradually filled up. 

 This fact, however, does not preclude the possibility of a previously 

 existing additional structure : it shows that, if there had been such 

 structure, it was either destroyed before the original substance of 

 the shell was replaced, or else that the shell was not fossilized in 

 such a manner as to preserve the inner parts. 



The form of the band bears a stroug resemblauce to some of the 

 Silurian shells referred by Lindstrom to his Alatce ^ division of 

 Pleurotomaria, This likeness is more especially remarkable in 

 PI. cirrTiosa Lindstr.,^ which also comes near in size, in the direction 

 of the lines of growth, and in the shape of the section of the whorls. 

 It must be noted, however, that Prof. Koken separates PJ. cirrliosa ^ 

 from the rest of the Alatce, because its whorls are quadrangular 

 instead of circular in section. Tropklostrojplia is distinguished by 

 having the keel less produced, in its being situated higher, and 

 never adhering to the succeeding whorl. The members of this 

 genus also resemble PI. limata Lindstr.,^ which belongs to his Incisce 

 group ; in it, however, the margins of the baud, though greatly 

 produced, do not unite, but always leave a furrow : moreover, the 

 lunules are very peculiar and distinctive in shape. The keel is 

 somewhat like that of TrocJms lundgreni Lindstr.,^ and also of some 

 of the genera allied to Troclms described by Prof. Pernor. The 

 genus, however, differs from all of these, as well as from Palceo- 

 troclms HalP and Eotrochus Whitfield," in the form of the aperture, 

 which does not recede below, in the higher position of the keel, in 

 its not adhering to the anterior whorl, and in the base being more 

 produced. 



When the outer sharp keel is broken off, it resembles some species, 

 of Mourlonia,^ to which genus L. G. de Koninck has referred it; 

 but it is distinguished by the structure of the keel, as well as by 

 the lines of growth being nearly vertical below. 



Another interesting feature is observable in the two species here 

 described as belonging to this genus, namely the presence of 

 numerous pittings scattered over the surface ; they occur in both 

 the outer and inner shell-layers, and their impression is left on the 

 internal mould ; the}' vary in size, being sometimes small and in 



^ ' Silurian Gtistropocla & Pteropoda of Gotland ' Kongl. Svensk. Yetensk.- 

 Akad. Handl. n. s. ^ol. xix (1884) tio. 6, p. 115. 



2 Ibid. p. 121 & pi. xi, figs. 27-29, pi. xii, figs. 1-3. 



' ' Uebei' die Entwickelnng der Gastropoden yom Cambrium bis zur Trias *■ 

 Neues Jabrb. Beilage-Band vi (1889) p. 438. 



^ Iv. Svensk. Yeteusk.-Akad. Handl. n. s. vol. xix (1884) No. 6, p. 114 & pi. x, 

 figs. 2-17. 



« Ibid. p. 149 & pi. xiv, figs. 46-53. 



« • Palseontologv of New York ' vol. v, pt. ii (1879) p. 133. 



' Eull. Amer. Mus. Nat. Hist. vol. i (1881-86) pp. 77-78. 



8 ' Faune du Calcaire Carbonifere de la Belgique' pt. iv, Ann. Mus. Eov. 

 Hist. Nat. Belg. vol. viii (1883) p. 75. 



