MISCELLANEA. 175 



with the ventral margin uppermost against the fi'ee margin of 

 the larger specimen at a, and thus the free margins are com- 

 pressed together and the original contour of the shell destroyed. 

 The hinge-margin is also more slightly arcuated, the umbones 

 more distinct, and the thickening between them and the posterior 

 margin more defined. I hope shortly to give a fuller account of 

 the geological position of the bed in which these shells were 

 found. The generic term Archanodon is proposed for this huge 

 freshwater bivalve. — Richard Howse, Newcmth-on-Tyne, August 

 4, 1877. 



Notes on a Paper hj R. H. Traquair, M.D., F.G.S., F.R.S., 

 on the Structure of the Lower Jaw in Rhizodopsis and Rhi%odus. — 

 In the First Volume of the Fourth Series of the Annals and 

 Magazine of JSTatural History, Yol. XIX., p. 299, were pub- 

 lished '']N'otes" on Rhizodopsis, by my late friend Mr. Albany 

 Hancock and myself, in which the prsemaxilla is described as a 

 long, narrow, wedge-shaped bone, with a row of small teeth 

 along the margin, and two laniaries ; one large, the other small, 

 the latter placed in front of the former, and near to the sym- 

 physis. 



Since then a large number of specimens of the cranium of 

 Rhizodopsis, several showing the prsemaxilla in situ, have been 

 obtained from the black shale at iJ^ewsham. 



From three of the largest of these the matrix has been care- 

 fully and entirely cleared away, so tJiat all the bones can be well 

 seen. In these specimens the maxilla and our prsemaxilla evi- 

 dently form the margin of the upper jaw, along which is placed 

 a row of small teeth, with two laniaries, one smaller than the 

 other and lying near to the symphysis, quite as in our description. 



The outline and the outer surface of the mandible are also 

 well defined, with the row of small teeth along its alveolar bor- 

 der interlocking with or being overlain by those of the maxilhi 

 and premaxilla. On upwards of half a dozen mandibles in my 

 collection, with their inner surfaces exposed, there is distinctly 

 seen the row of small teeth along tlieir upper margins, with the 

 large laniary in front and the other three laniaiies behind ; tlieso 



