248 Messrs. Davidson and Kino; on the Genera 



& 



XXXVII. — Remarks on the Genera Trimerella, DinoLolus, 

 and Monomerella. By Thomas Davidson, F.H.S., F.G.S., 

 &c., and William King, Sc.D., 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 

 Palliohranchiata or Brachiojjoda. We propose to designate 

 it TrimerellidcB, 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 sup- 

 plied us with the loan of some valuable series of specimens, and 

 presented us with others, to undertake the 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 

 "Remarks," 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 determination. We think there is little doubt of their being 

 not only structurally related to the Lrngulidai *, but also gene- 

 tically connected with this family. The first point is of con- 

 siderable interest, inasmuch as the Lingulids are the earliest 

 Palliobranchs that geologists are acquainted with, occurring 

 in Cambrian rocks ; while the Trimerellids 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 modificationst), 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 and Rhynchonellids that suc- 

 ceeded them, the conclusion suggests itself that the latter 

 and simpler groups are the degraded successors of a type 

 that existed in the earliest known Life-period of our planet. 

 Another matter for consideration is the fact that the Cambrian 

 Lingulids were furnished with a framework of a horny or 



* For the present we include Oboh/s and other related genera in the 

 Litifiulidcp — though we nre strongly inclined to regard the genus named 

 as typical of another family, Oholidce. 



t" See ' Geologist/ vol. v. p. 2-j4. 



