82 G. LINDSTRÖM, ON TIIE SILURIAN GASTROrOlJA ANI^ rXEKOPODA OF GOTLAND. 



1 p. 147 shown that it in no essential characters differs from Cyrtolites. Troijidiscus 

 Meek, or Tropidodiscus as the name has been einendated by Waagen, is one of those 

 forras in which the whorls are more nuraerous and involute than the others and the 

 dorsal carina thin. As the nanie had already been preoccupied for a recent shell of 

 the Planorbida3, De Koninck has instead proposed Tropidocydus, but this as well as 

 the former ones must be regarded only as synonyms to Cyrtolites. Porcellia Verneuili 

 D'Orb., De Kon. Faune Carbonifére, II, pt IV, 288, belongs probably also to the genus 

 Cyrtolites. 



There is some difference as to the involution and number of the whorls, and as 

 to the size and sculpture of the dorsal carina. But the common compressed, discoid, 

 shape and the form of the aperture, which as to its details is nearly alike in all, are 

 sufficient grounds to unite them in the same genus. 



The geological range of its species is limited in Sweden to the Upper Silurian 

 formation of Gotland from where five species are known in the shale and sandstone 

 beds, and three in the superjacent limestone, one species being common to both. Abroad 

 Cyrtolites continued as far as now is known in both the Devonian and Carboniferous 

 formations. In North America several species are found already in the Lower Silurian. 



Conspectus of the species: 

 I. Whorls free or scarcely contiguous, few in number. 



1. C. lamellifer n. 



2. C. pharetra n. 



II. Shell with contiguous whorls, numerous, disciform. 



3. C. arrosus n. 



4. C. obliquus n. 



5. C. euryomphalus n. 



6. C. discus n. 



7. C. orbiculus n. 



1. Cyrtolites lamellifer n. 



Pl. VI ligs. 31—38. 



Shell laterally compressed, along the median line of the sides somewhat tumid 

 and thence elliptic in a transverse section. The surface is transversally ridged by a 

 succession of regularly distantiated, outstanding lamella3, wide apart at the dorsal side, 

 converging towards the ventral side and most of them meeting. They stånd out, 

 slightly bent against the aperture of the shell (figs. 34, 37), looking like hooks when 

 seen edgewise. They are about fifteen and represent the successive apertural borders. 

 They are crossed by regular, elevated longitudinal striaj nearly of equal size, separated 

 by interspaces of the same width. Rectangularly to them, minute threadfine ornamental 

 lines are running. The dorsal keel or the slit band is moderately elevated and in 

 direct continuation of a deep tongue shaped slit in the middle of the superior lip of 

 the aperture. The slit band is even and consists of small arched lines (figs. 32, 38), 



