88 G. LINDSTRÖM, <)N THE SILURIAN GASTUOVODA ANIJ l'TEUOrO])A OF GOTLAND. 



of largest whorl at the aperture 32 millim. Height of the same 18 millim. Occurs 

 in the liinestone beds of Östergarn, but only as nuclei, also at Hammaren in Kräklingbo. 

 Amongst formerly knovvn species, Bellerophon Aymestrensis So\y. Sil. syst. p. 

 61(5, pl. 6 fig. 12 seems to be nearest related to this species, but there are no apertures 

 on its hack, as far as the evidence goes. 



Fara. VI. PLEUROTOMARID^ D'Orbigny. 



Since Dall in the Bulletin of the Museum of Comparative Zoology, vol. IX, M 2 

 (1881) p. 77 and following, made known the i^esults of his researches on the structure of 

 the animals of two different species of Pleurotomaria, dredged up from the depths of 

 the Mexican Gulf there are more valid grounds than before to attribute to the members 

 of that large genus an independent position in a family of their own. Scissurella which 

 had long been considered as the only living representative of the Pleurotomaridae, 

 must henceforth be separated from them and receive its place with the Trochidas, while 

 Pleurotomaria, according to Dall »stånds nearest the Trochida?, with features recalling 

 the HaliotidaB». This same opinion had indeed also been pronounced by sorae other 

 zoologists previous to Dall's researches on the animals themselves, as by Claus, though 

 he includes Scissurella in this group. 



It is, however, very difficult, on various grounds, to draw the exact limits in 

 respect to several of the palajozoic forms and to tell with certainty, which are belon- 

 ging to this family and which are not. It must be borne in mind that there are se- 

 veral shells, besides the Pleurotomarise and belonging to quite different families, which 

 are provided with a slit in the exteriör lip and a slit band, as nearly as possible re- 

 sembling that of Pleurotomaria. Scissurella, for inst., is provided with a similar band 

 and also Emarginula, in both of which the organization of the animals is different from 

 that of Pleurotomaria. In the Bellerophontidaj there also occurs a slit band, which in 

 many of them, though not in all species, is formed upon the same plan as in Pleuro- 

 tomaria. 



The peculiarity in the structure of the body, which has caused the deep slit, 

 may, no doubt, be shared by several other Gastropoda, else dissimilar, but in them 

 originating only a shallow notch, as sometimes, not typical, in certain specimens of 

 Platyceras or typical in all species of several genera, as in the next family, the Euom- 

 phalidte or in the large genus Pleurotoma or in Siliquaria. In none of these, however, 

 the peculiar slit band is marked by distinct crescent-formed lines of growth or la- 

 mellte, sometimes attaining an enormous development and enclosed within two or even 

 more distinct elevated lines or lamellaä. 



The nacreous shell of the true Pleurotomaria? is still preserved in a great ma- 

 jority of the Silurian species which are described further on. There must rest some 

 doubt concerning those, in which the nacre is not left behind, but which else in all 

 details are like the Pleurotomaridos and which, provisionally at least, may be included 

 in this family. 



