liONGL. .SV. VKT. AKADEMIENS IJANUL. IJAND. 19. N:0 6- 53 



bracliiopoda, thoiigli they did not j)vd)lish their o])inioii. When we, however, coinparc 

 one of the species, Tr. unguis, witli recent forms of the genus Patelhi, it is quite evi- 

 dent that it botli on exterior and interior grounds should be numbered amongst the 

 Gastropoda and especially in the family of the Patellidai. In plate I, fig. 38 there is 

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

 exterior shape, sculpture and colouring as nearly as possible resemhles Tr. unguis fig. 

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

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

 difit'erences in the details, the same disposition of tlie six or seven pairs of those im- 

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

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

 as the Jjingula3 and Granite amongst the Brachiopoda and Calostylis amongst the Co- 

 rals have continued till our time with small or insignificant 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 

 tlie oldest form, >^FatelUv^ antiquissima, from the Swedish Lower Silurian, being ap- 

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

 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 Scutellida?. As in Tryblidium the foremost 

 [)air is also the largest with the recent Patellar, but there is the great distinction that 

 both these last muscles of the living Patella; 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. Gharacteristic 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 Patelke has thus consisted in the concen- 

 tration of the once detached and entirely independent muscular scars. 



In the »Pala30zoic 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 exterior 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 Metoptoma\ Meek and Worthen') make the ade- 

 quate remark, that later authors have given to that genus a greater extension than PiiiL- 

 Lirs 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 difl'erentiate 

 these two genera, as there in Metoptoma, for inst. ^i. pileus Phillips, according to De 



') Proc. Acad. Nat. Sc. Philadi-li.liia, 18G6, p. 266. 



