54 G. LINDSTKÖM, UN TIIE SlLUlilAN GASTROPODA AN]) PTEllOPODA OF GOTLAND. 



KoNiNCK ill Descript. Anini. foss. de Bclgique Suppleiii., p. 685, pl. 58, liy. 1, exists 

 a coherent inuscular sear, horsc shoe shaped, with the apex of" tlie shell towards its 

 bind side ^), whereas in Tryblidiuin there are six pairs of detached scars, and the apex 

 anterior. 



From the upperuiost beds of the Lower Silurian rocks of Esthonia, at Borkholm, 

 I hava seen specimens of two species belonging to this genus, true forerunners to the 

 two Upper Silurian ones; of these one can scarcely be distinguished from Tr. unguis, 

 the other again very elosely resembles Tr. reticulatum. From Raikiill in Esthonia, in 

 Friedrich Sciimidts stratum G 3, at the base of his Upper Silurian there occurs also 

 a third species, allied to Tr. unguis, but regularly, transversally irabricated by equi- 

 distant lines of growth, and of an almost elliptical outline. 



Mr I. F. Whiteaves, of the Geol. Survey of Canada, has recently^) described a 

 »Tryblidiura Canadense» from the Guelph formation of Canada. In respect to its ex- 

 teriör shape it might well be united with the Tryblidia, though it is more elevated, 

 but the figures of pl. V, f. 1 & la show a narrow, continuous ring of inuscular im- 

 pression, quite unlike what prevails in the other Tryblidia and, moreover, there are 

 below the apex two oval depressions different from anything seen in the Tryblidia. Till 

 these points are cleared up, the Canadian species can only with hesitation be placed in 

 that genus. Whiteaves also thinks that Metoptoma Niobe, M. Nycteis, M. Eubule, M. 

 Erato and M. Hyrie Billings are typical species of Tryblidiuin. This can, however, not 

 be finally settled, before the inuscular impressions shall liave been discovered. 



Metoptoma Erato Billings '»Palajozoic Fossils of Canada» vol. 1 p. 39, is near to 

 Tr. unguis, but probably identical with the Esthonian species, as it is derived from the 

 Lower Silurian stratum called »Black River Limestone». Nearly allied to this genus is 

 also the Lower Silurian »Patella antiquissima» described by Hisinger and formerly 

 found at Borenshult in the Retiolites shale. In outward shape it reminds of Palagac- 

 masa Hall, and it is also like several of the MetoptomaB, but the six pairs of most 

 beautifully preserved muscular scars place it near Tryblidium. 



As to the first appearance of this group within the palasozoic era Ihering says 

 in his »Anatomie und Phylogenie der Mollusken», p. 82, »ist doch so viel sicher, dass 

 die pala30zoischen Patelloideen Tecturiden vvaren», and further: »in der Primordialfauna 

 sind schon die Tecturiden und Lepetiden vertreten», and he seems to confound the 

 Patellida3 with the Patelloideoi, as he numbers Patella vulgata amongst them, nor does 

 he anywhere mention the occurrence of the Patellida3. But through the occurrence of 

 the Tryblidia and other kindred genera, already present in the Lower Silurian we find, that 

 the Patellidai were also represented by numerous, well developed shells and consequently 

 that both families, the Tecturida; and Patellida3 coexisted. 



') NicHoLsoN again in his Handbook of Palaioutology 11 p. 34 says that iu Metoptoma »the muscular scars 



consist of a number of disconnected cavities». 

 2) Geol. & Natural History Survey of Canada. Palieozoic Possils, vol. III, pt 1, pp. 30—31, 1884. 



