434 Mr. T. Davidson on the Brachiopoda. 



follow, and the family to which this last-named genus gives its 

 name be completely dispensed with. 



In the sixth, seventh and eighth families we have made no 

 further changes than to entirely expunge D'Orbigny's genus 

 Orhiculoidea, there existing at present no valid grounds for its 

 adoption. 



Tt is difficult to see much order or interdependence in the suc- 

 cession of groups. It seems that the hingeless genera, Lingula, 

 Obolus, Crania, Discina and Produdus, which commenced in the 

 earliest sera, soon attained their climax, since only three of them, 

 Crania, Discina and Lingula, have passed the limits of the 

 palaeozoic age. They are also least unHke the Lamellibranchs 

 {e.g. Anomia, Hippurites, Spondylus). Of the hinged genera 

 those provided with calcareous spires were certainly developed 

 first, no example having as yet been found above the lower por- 

 tions of the Jurassic strata; while, on the contrary, the genera 

 provided with loops become important in the Jurassic period 

 and continue up to the present time. For in all the Palaeozoic 

 epoch we are acquainted with but few species which possessed 

 loops, and until M. Suess's discovery of the interior of T. Archiaci 

 no species with a long loop had been discovered, if we except 

 String cep halus. 



Thus taking our present Table as a basis for our conclusions, 

 we find the genera and subgenera distributed in time as follows : 



In the Silurian 20. In the Devonian 25. In the Carboni- 

 ferous 19. In the Permian 12. In the Triassic 12. In the 

 Jurassic 14. In the Cretaceous 12. In the Tertiary 10. In the 

 Recent 14. 



These details vary but little from those published in 1853, but 

 it is to be expected that future discoveries will still slightly 

 modify our statistics, as there are many obscure forms which 

 have hitherto resisted our investigation. 



It would take up too much space to enter here upon the 

 numerous additions made to the two foreign editions, but we 

 cannot pass in silence the interesting discovery of the perfect 

 interior of that shell known under the name of Rhyjichora David- 

 soni (De Koninck), and of which the Anomia spathulata of 

 Wahlenberg is another similar form, this last being the second 

 type of Dalman^s objectionable genus RhyncJiora^. During the 



* Dalman proposed his genus Rhynchora ia 1827, with the following 

 diagnosis : — " Smaller valve truncated at its base, larger valve with a nearly 

 straight? elongated beak : Rhynchora costafn (Wahl.), Rhyn. spathulata:" 

 but it is certain that neither of those shells has a lengthened beak like 

 Lyra ; and if Dalman intended to refer his species to Cumberland's genus 

 (as Hisinger would lead us to believe), he was there again in error, as it is 

 certain that they do not even belong to the group. 



