448 M.R. S. S. BTJCKMAN ON THE BAJOCIAN [Aug. 1895, 



ribs, and its lateral folds about seven ribs, witb several smaller 

 towards the beak-area ; margin of valves on tbe beak-area consider- 

 ably curved. 



This fine RhynchoneUa lived during the Murchisonce hemera. It 

 is a mistake to identify it with Waagen's gingensis, which, though 

 a shell of somewhat similar appearance in general, is much less 

 gibbous, especially in the imperforate valve, and has a more 

 distinctly separated mesial fold more numerously ribbed, and is on 

 the whole a less rotund shell. It lived several hemerse later, and 

 may be a descendant of the present species. 



The name now given is suggested by the swelling imperforate 

 valve. The species is excellently depicted by Davidson. It occurs 

 at Bradford Abbas, Dorset, and is rather scarce. 



(d) The Rhynchonella-cynocephala Group. 



The top of the Oolite Marl Series (Upper Freestone) is an un- 

 certain datum-line, owing to the different denudations to which it 

 has been subject ; yet this top has played an important part in 

 the present paper, because between it and the Upper Trigonia-giit lie 

 the ' intervening beds,' when they are present. But if this top of the 

 Upper Freestone is an uncertain datum-line, it becomes necessary 

 to take a palaeontological datum-line. Such is afforded by a species 

 which I have named RhynchoneUa cynomorpha, a form occurring 

 latest of any of the brachiopods of the Oolite Marl Series. 



It has been recognized for some time that this species required 

 distinction from RhynchoneUa cynocephala ; but before its differ- 

 ences therefrom could be properly stated it was necessary to know 

 exactly what that species was. The consequence of such investi- 

 gation is to find several species that have been confounded under the 

 name RhynchoneUa cynocephala, while that species itself was, perhaps, 

 masquerading under another name. As the species belong to 

 different horizons, and as they could be used for the identification 

 of such horizons, it becomes necessary to review the whole of them. 

 Their stratigraphical positions are shown below, as well as their 

 generic connexion among themselves and with other fossils. The 

 course of development is increase of plication and decrease in 

 elevation of mesial fold. Thus the series may be traced back to 

 RhynchoneUa acuta of the Marlstone (Middle Lias), and forward to 

 RhynchoneUa subangulata. The RhynchoneUa acuta, has an uni- 

 plicate mesial fold ; Rh. cynoprosopa and cynocephala have generally 

 triplicate folds — the 1 and 3 forms rare ; Rh. cynica has no 1-fold 

 form, the 2 and 3 forms common, and the 4 form rare ; while Rh. 

 subangulata has 4, 5, and 6-plait mesial folds — the fewer-fold form, 

 according to Mr. Upton, being geologically earlier. 



