442 Davidson and King — On the Trimerellidce. 



not being mucli disturbed before the deposition of the Upper 

 Sihii'ian, for there is not the slightest passage, either stratigraphically 

 or palcBontologically, from the Coniston Limestone series into the 

 lower division of the Upper Silurian, which is the Stockdale Shales 

 (including the G-raptolitic Mud-stones), and these are equivalent to 

 the Tarannon Shales or Pale Slates of North Wales. It is also seen, 

 on a close survey, that the Stockdale Shales will occasionally par- 

 tially overlap the Coniston Limestone series, and in one locality, I 

 believe, completely ; it will be also noticed that the strikes of the 

 lower and higher series of beds do not always correspond. 



III. — Eemakks on the Genera Tbimerella, Dinobolus, anb 



MONOMEBELLA. 



By Thomas Davidson, F.E.S., F.G.S., etc., and "William King, Sc.D., and 

 Professor of Mineralogy and Geology in Queen's College, Galway. 



THE genera named in the title constitute in our opinion a new 

 family belonging to the helictobrachial section of the class 

 PalUohrancMata or BracMo'poda. We propose to designate it Trime- 

 rellidce, after the type genus. 



Although more or less treated of by other writers, we have been 

 induced, especially by the desire of several intimate friends, who 

 have kindly supplied us with the loan of some valuable series of 

 specimens, and presented us with others, to undertake a further 

 elucidation of a most difficult and enigmatical group of shells ; and 

 for this assistance our thanks are especially due to Lindstrom, Walm- 

 stedt, Billings, Hall, Whitfield, Meek, and others. 



These " Eemarks," it is necessary to state, are merely preliminary 

 to a detailed memoir we have been preparing for some time past ; 

 and which we hope to have completed for the Geological Society in 

 the early part of next session. 



The Trimerellids differ much from all others of their class ; though 

 their proximate alliance to certain forms seems to admit of determi- 

 nation. We think there is little doubt of their being not only 

 structurally related to the Lingididce ;^ but also genetically connected 

 with this family. The first point is of considerable interest, inas- 

 much as the Lingulids are the earliest Palliobranchs that geologists 

 are acquainted with, occurring in Cambrian rocks ; while the Trime- 

 rellids do not seem to have been in existence prior to the next 

 systemal group, all the forms belonging to the Lower and Upper 

 Silurians. It would therefore appear that the Trimerellids, adopting 

 the doctrine of genetheonomy (by which we mean evolution of 

 species effected mainly through the operation of Divine laws, and 

 not by purposeless or accidental modifications ^), have been produced 

 out of the Lingulids. Moreover, considering that the earliest 

 Palliobranchs, taking them to be represented by the existing aniferous 

 Lingulas, are of a simpler type than the non-aniferous Terebratulids 



^ For the present we place Obolus and other related genera in the Lingulida. 

 This does not prevent our regarding Obolus as typifying another family ; but the 

 allocation now adopted simplifies these " Remarks." 



^ See Geologist, vol. v. p. 25i. 



