30 G. LINDSTRÖM, ON THE SILURIAN GASTEOPODA AND PTEKOPODA OF GOTLAND. 



Sweden. Upwards, into the Devonian strata of France and N. America, oiily the fol- 

 lowing species is passing, viz. Pleurot. labrosa Hall, also found in the »Devonieii in- 

 férieur» at Néhou and in North America in the Lower Helderberg rocks. Plntyceras 

 cornutum His. can in some of its varieties scarcely be distinguished from the Eifelian 

 Pileopsis prisca to which it at all events stånds in very close relation. Bellerojihon 

 trilobatus Sow. also seems to be a species which has continued in the Devonian time. 



Zoological charaeters of the gastropodan fauna. 



If we except Chelodes and the genera Subulites, Euchrysalis and perhaps Onycho- 

 chilus, all the other belong to the large section, which on account of the circular 

 peristome has been called that of the Holostomata. Chelodes as one of the Chitonidge, 

 belongs to the stränge order Placophora. Subulites, Euchrysalis and possibly also 

 Onychochilus again exhibit such charaeters in their aperture as to entitle us to place 

 them with the great order Siphonostomata, hitherto regarded as of uiesozoic origin 

 and entirely unknown in the palaäozoic formations. It cannot be denied that the evi- 

 dence offered further down supports this conclusion and that it probably shall be 

 corroborated by further observations. If then, as I suppose, Subulites is one of the 

 Siphonostomata, these have originated much earlier than in the Gotlandic Silurian, as 

 there are well preserved species of that genus in the Lower Silurian strata of Dale- 

 carlia. Subulites nitens Ldji (in Fragmenta Silurica p. 14) occurs already as early as 

 in the Lower Gray Orthoceratite Limestone of Sjurberg, where besides this no other 

 forms of the same order are known. A larger species, probably Subulites (Helicites) utri- 

 cularis Wahlenbeeg (Subul. elongatus Portl. according to Fragm. Silur. p. 13), is found 

 in the uppermost stratum of the Lower Silurian of Dalecarlia, the Leptasna limestone. 



Of the fifteen families represented, as many as twelve are also recent. It must, 

 however, be conceded that it is with great diffidence that several families, such as the 

 Litorinida?, the Pyramidellida?, the Turbinidse have been introduced in the lists. Of 

 the 25 genera again, the majority or 23 are considered to be extinct and only two, 

 Pleurotomaria and Trochus, still persisting. The former genus, rich as it was in the 

 Silurian times, had not then reached its acrae, which was attained in the Jurassic seas 

 and now it has dwindled away to four species in the recent seas. Together with Mur- 

 chisonia it gives to the fauna of the Gotland Gastro[)oda its chief character, their 54 

 species making nearly a third of the whole fauna. Like Pleurotomaria there are other 

 genera, the systematic place of which cannot be contested. The beautifully pi'eserved 

 muscular scars on the inner side of Tryblidium certify their near connection with the 

 recent Patellidfe. Chelodes with its numerous very characteristic plates belongs in all 

 probability to the Chitonidse. The numerous Oriostomata, forming such a prominent 

 feature in the fauna, testify through their operculum and their nacreous interiör strata 

 to their relation with the Turbinidas. It is also very likely that the stränge Autodetus 

 is an ancestral form representing the modern Phoridte, the presence of which 

 family has already been observed in the Devonian strata. But instead of attaching 

 other objects to its shell as its presumed descendants, it fixed its own shell with the 

 apex to other bodies. Of the occurrence of such an archaic from as Dentalium, nothing 



