KONGL. SV. VET. AKADEMIENS IIANDL. BAND. 19. N:0 6- 53 



brachiopoda, thoiigb they did not piiblish their opinion. VVhen we, however, couipare 

 one of the species, Tr. iinguis, with recent forms of the genus Patella, it is quite evi- 

 dent that it both on exteriör and intei^ior »rounds should be numbered amongst the 

 Gastropoda and especially in the fainily of the Patellida). In plate I, fig. 38 there is 

 delineated a recent species of the subgenus Nacella Adams from Foiia, which as to its 

 exteriör shape, sculpture and coloui'ing as nearly as possible resembles Tr. unguis fig. 

 33. And again if we turn to the interiör muscular scars and compare figures 30 and 

 37 on the same plate with Patella cochlear L. fig. 32, there is, if we except some 

 differences in the details, the same disposition of the six or seven pairs of thosc im- 

 pressions. The species of this Silurian genus Tryblidium, and especially, such a form 

 as Tr. nnguis may then be added to the so called persistent types of Huxley, which 

 as the Lingulaj and Cranias amongst the Brachiopoda and Calostylis amongst the Co- 

 rals have continued till our time with small or insigniiicant modifications in their 

 structure. It is intermediate between the recent genera Olana and Nacella. 



In the Silurian species the scars are disconnected and deep, the more so in 

 the oldest form, ^^Patellcv^ antiquissima, from the Swedish Lower Silurian, being ap- 

 proached, though separated, in the Upper Silurian species. In the recent Patella;, 

 the scars are in most species nearly connected, though it, especially on their inner 

 margin, is possible to discern how they have been independent. This is the more evi- 

 dent, when we turn to the animal itself, where muscular pairs of variable number in 

 the same species, from six to nine, are distinctly detached and free at least partially 

 and do not form a continuous ring as in the Scutellidaj. As in Tryblidium the foremost 

 pair is also the largest with the recent Patella3, but there is the great distinction that 

 both these last muscles of the living Patella3 are united through a narrow stripe of 

 muscular tissue, forming an arched curve, also visible as a scar on the shell, whereas 

 in Tryblidium the narrow stripes which are emitted from the large, foremost scars do 

 not unite, but leave an open space between them. Characteristic to the foremost pair 

 in Tryblidium is also the large appendiculated scar on their inner margin, of a 

 peculiar reticulation, described more in detail below in Trybl. unguis. The change 

 in this respect from the older to the recent Patellaj has thus consisted in the concen- 

 tration of the once detached and entirely independent muscular scars. 



In the »Palseozoic Fossils of Canada» Billings has described a species of Metop- 

 toma, M. Hyrie, p. 87, fig. 79, which comes near to Tryblidium unguis as far as may 

 be judged by the exteriör appearance. It is evident that Billings in the cited work 

 has enclosed within that genus too many species which do not show the characters 

 given by its author Phillips in »Geology of Yorkshire» pt. 2, p. 223 and that conse- 

 quently only some of them are true Metoptomaj. Meek and Woethen^) make the ade- 

 quate remark, that låter authors have given to that genus a greater extension than Phil- 

 lips intended. The new genus which I have proposed, differs from Metoptoma in wan- 

 ting the broad truncated area below the apex. The muscular scars also differcntiate 

 these two genera, as there in Metoptoma, for inst. M. pilens Phillips, according to De 



1) Proc. Aoad. Nat. So. Philadelpliia, 1866, p. 266. 



