﻿CRETACEOUS BRACHIOPODA. 



01 



In 1855 I followed d'Orbigny and some other Palaeontologists while referring the 

 shells PI. XI, figs. 1 — 5, and PI. XII, fig. 25, to one of the supposed varieties of 

 Bh. compressa, Lamarck, sp. D'Orbigny observes, at p. 37 of his ' Paleontologie Erancaise,' 

 that there appear to be two varieties of Bh. compressa in the Upper Greensand of 

 Le Mans — one, depressed (Lamarck's type) ; the other, narrower and more convex ; 

 a more transverse variety of the same being likewise found at Havre, which would 

 hold an intermediate position between the two varieties occurring at Le Mans. 

 In England we do not find the depressed form of the Mans or true T. compressa 

 of Lamarck, our species being exceedingly variable and at the same time identical 

 with that occurring at Havre ; it is the T. dimidiata, Sow. =: T. alata and gallina, 

 Brongniart = T. dilatata, Sow., &c. 



Therefore, in order to avoid further confusion, it may be advisable to retain the name 

 dimidiata, Sow., for the forms to which I had applied the name of compressa, 

 and to add the varietal designation of convexa (Sow.) to those specimens, from Warminster, 

 referred to Bh. compressa, Lamarck ; but, at the same time, it must be admitted that 

 intermediate forms may be collected in the same locality, which seem to connect, to a 

 greater or lesser extent, the varieties dimidiata and convexa. 



81. Rhynchonella dimidiata, Sow. Dav., Cret. Mon., PI. XI, figs. 1 — 5; PI. XII, 



fig. 25. 



Rhynchonella compressa, Dav. Cret. Mon., p. 80, pi. XI, figs. 1 — 5 ; pi. XII, 



fig. 25. 



This would agree well with Bh. dimidiata, Sow. {Bh. alata and gallina, Brong.). When 

 symmetrical, it is the form to which the last two names had been given, and when distorted 

 or unsymmetrical it would be the shell described as dimidiata, Sow. Many species of 

 Bhpichonella have their valves occasionally unsymmetrical, the mesial fold becoming 

 totally or partially shifted either to one side or the other. This is often the case 

 with Bh. sulcata, Bh. difformis, Bh. lata or latissima, Bh. antidichotoma, and many other 

 forms. When perfectly well shaped, Bh. dimidiata is transverse and laterally expanded. 

 The surface is marked with about forty strong ribs, of which some nine or ten form a 

 wide, slightly raised, but flattened, mesial fold. 



This species occurs in the Upper Greensand of Warminster ; in the cliffs near Lyme 

 Regis, at Beer Head, &c. ; in the Chloritic Marls or Discoidea- and Scaphites-beds 

 of Chard and Chardstock ; in the Upper Greensand of the Undercliff, Isle of 

 Wight, &c. 



