54 G. LINUSTROM, UN TllK SILUKIAN GASTUoJ-ODA AND I'TEUOPUDA OF GOTLAND. 



KoNiNCK ill Dcscript. Aiiiiu. loss, dc liulgiquu Suppleiii., p. 685, ])1. 58, lig. 1, exists 

 u coherent iiiusculur sear, horse shoe shaped, with the apex of the shell towards its 

 hind side'), whereas in Tryhlidiuui there are six pairs of detached sears, and the apex 

 anterior. 



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

 I have 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, 

 tlie other again very closely resembles Tr. reticulatum. From Raikidl in Esthonia, in 

 FniEnmcii Schmidts stratum G 3, at the base of his Upper Silurian there occurs also 

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

 distant lines of growth, and of an almost elli[)tical outline. 



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

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

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

 but the figures of pi. V, f. 1 & la show a narrow, contiiuious ring of muscular im- 

 pression, (juite unlike Avhat 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, tiie 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 Tryblidium. This can, however, not 

 be finally settled, before the muscular impressions shall have been discovered. 



Metoptoma Erato Billings 'nPalasozoic 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 Pala^ac- 

 mtua Hall, and it is also like several of the Metoptonuij, but the six pairs of most 

 beautifully preserved muscular scars place it near Tryblidium. 



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

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

 die pala3ozoischen Patelloideen Tecturiden waren», and further: »in der Primordialfauna 

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

 Patellidte with the Patelloidete, as he numbers Patella vulgata amongst them, nor does 

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

 the Tryblidia and other kindred genei-a, already present in the Lower Silurian we find, that 

 the Patellida^ were also represented by numei'ous, well developed shells and consequently 

 that both families, the Tecturidaj and Patellidaj coexisted. 



') Nicholson again in liis Handbook ol' Pahtoutology II P- ^i says that in Metoptoma "the muscular scars 



consist of a number of disconnected cavities". 

 -) fieol. & Natural History Survey of Canada. Paheozoic Fossils, vol. Ill, pt 1, pp. 30 — 31, 1884. 



