Geological Society of London. 233 



year Mr. Billings established the genus Monomerella. Bhynoholus 

 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, Dinobolus, and Monomerella as 

 constituting their family Trimerellidse. 



The Trimerellidae are described as having a calcareoiis 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 dentated ; 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 Trimerellid^, that they are most nearly allied 

 to the Lingulidce, although presenting characters of sufficient im- 

 portance to warrant their separation as a distinct family. 



As far as is at present icnown, the TrimerellidcC are confined to the 

 Cambro-Silurian and Silurian systems ; but the authors indicate the 

 probability that in the Cambrian or early Cambro-Silurian rocks a 

 generalized form will hereafter be detected, bringing Discina, Lingula, 

 and Obolus iuto close relationship, as regards their myology, with 

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

 root from which the Trimerellidee 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 Zindstromi, Dall ; 4. T. Billingsii, Dall ; 

 5. r. (?)(jraZiensjs, Bill., = mmor, Dall; 6. T. OMoe?zsis, Meek ; 7. T. 

 Dalli, Dav. and King ; 8. T Wisbyensis, Dav. and King. 



Genus 2. Monomerella, Billings (1871). Sp. 1. M. prisca. Bill.; 

 2. Af. Walmstedti, Dav. and King ; 3. M. Zindstromi, Dav. and King ; 

 4. M. orbicularis, Bill. 



Genus 3. Dinobolus, Hall (1871). Sp. 1. D. Conradi, Hall; 

 2. D. Davidsoni, Salt.; 3. D. Canadensis, Bill.; 4. D. transversus. 

 Salt. ; 5. D. Woodvmrdi, Salt. ; 6. D. magnificus, Bill. ; 7. D. Schmidti, 

 sp. n. 



The authors further discuss the characters of Professor Hall's 

 Lingulops Whitfieldi, which they regard as in some respects inter- 

 mediate between the Lingulidae and Trimerellidse, and describe under 

 the name of Chelodes Bergmani, a fossil sent to them by Dr. Lind- 

 strom under the impression that it was a Trimerellid. They indicate 

 its points of resemblance to Trimerella, but think that it really 

 belongs to the section of the Coelenterata represented by Calceola 

 and GoniopTiyllum. The fossil is from the Isle of Gothland, and was 

 obtained from a formation equivalent to the Aymestry limestone. 



Discussion. — Mr. Hicks remarked that the oldest known Lingulid is Lin 

 primceva. The LingulcB in the earliest rocks increase in size as they approach shallow 

 deposits, and higher up, namely in the Lingula-flags, a change takes place in their 



