76 Geological Society. 



at least 3000 feet in Tertiary times, during which the Miocene 

 deposits were formed. The Tell plateau was thus elevated at least 

 4000 feet, and the district north of the Lesser Atlas at least 1000 

 feet, the north face of those mountains probably marking a post- 

 tertiary line of fault of 3000 feet. This operation was followed by 

 a long period of denudation, and this by a post-tertiary depression, 

 which the author terms the " Sahara submergence,*' after which the 

 land was reelevated at least 3000 feet, but perhaps considerably 

 more. A gradual subsidence appears to be still taking place. 



2. "On the Trimerellidae, a Palaeozoic Family of the Palliobranchs 

 or Brachiopoda." By Thomas Davidson, Esq., F.R.S., F.G.S., &c, 

 and Prof. William King, Sc.D. 



In this memoir the authors describe in detail certain Brachiopoda, 

 for which they propose to establish a distinct family, discuss the 

 characters and affinities of the family, and indicate certain geological 

 considerations which arise from their study of its members. The 

 first known species were described in 1853 under the names of 

 Obolus Davidsoni and O. transversus ; but in 1862 Mr. Billings de- 

 scribed an allied form as constituting a new genus called Trimerella. 

 With this Goilandia of Dall (1870) is identical. In 1871 Prof. Hall 

 proposed the new genera Rhynobolus and Dinoholus ; and in the 

 same year Mr. Billings established the genus Monomerella. Rhyno- 

 bolus is regarded by the authors as a synonym of Trimerella ; and 

 several other proposed genera being similarly cancelled, the authors 

 retain only the genera Trimerella, Dinoholus, and Monomerella as 

 constituting their family Trimerellidse. 



The Trimerellidse are described as having a calcareous shell, 

 generally massive, with the umbo of the pedicle-valve often large, 

 pointed, solid or hollow ; area usually of considerable size, with a 

 large solid deltidium ; hinge of both valves slightly dentary ; in 

 the interior of each valve there is a more or less elevated platform, 

 longitudinally placed, and either solid or doubly vaulted; and from 

 the middle of its anterior end a median plate occasionally projects 

 into the anterior half of the valve, especially in the brachial valve. 



In discussing the affinities of the new family, the authors enter 

 in considerable detail into the arrangement and functions of the 

 muscles of the Brachiopoda, and show, from the muscular indications 

 and other characters of the Trimerellidse, that they are most nearly 

 allied to the Lingulidae, although presenting characters of sufficient 

 importance to warrant their separation as a distinct family. 



As far as is at present known, the Trimerellidas are confined to the 

 Cambro- Silurian and Silurian systems; but the authors indicate 

 the probabilitj- that in the Cambrian or early Cambro- Silurian 

 rocks a generalized form will hereafter be detected, bringing Discina, 

 Lingula, and Obolus into close relationship, as regards their myology, 

 with each other and with Dinobolus. Such a form would constitute 

 the root from which the Triinerellidas have originated. 



The genera and species described are as follows : — 



Genus 1. Trimerella, Billings (1862). Sp. 1. T. grandis, Bill.; 

 2. T. acuminata, Bill. ; 3. T. Lindstrbmi, Dall ; 4. T. BilUngsii, 



