Counties of Durham and Northumberland. 311 



The examples from this district agree very accurately with the 

 original specimens described by Capt. Brown. In general, all 

 the specimens are strongly marked with several oblique radiating 

 ribs or planes over the posterior surface. Some specimens occur 

 at Byer^s Quarry quite smooth, arid more ovate than usual. 

 These have, I see, been prospectively named Pleurophorus ovatus 

 in the 'Perm. Mon/ They do not appear to differ specifically 

 from the foregoing. 



It ranges through all the limestone series, but is most plen- 

 tiful in the shell-limestone of Tunstall and Humbleton. 



29. Myoconcha MODioLiFORMis, King. — It has been pro- 

 posed by Baron Schauroth to unite this species with the Modiola 

 Pallasi, De Vern., a species which is said to be without teeth. 

 Mr. King has also referred it to the edentulous genus Cardio- 

 morphdy De Koninck. For want of Russian specimens to com- 

 pare with it, I think it better to leave it under its present specific 

 name, especially as some specimens which I have examined show 

 an appearance of a tooth in the right valve, and its other general 

 characters are the same as in the preceding species. It may 

 with safety therefore be included in the same genus as costata. 



In one of Mr. Kirkby^s specimens, the ligament, which is 

 finely preserved, is comparatively large. Some specimens of 

 the same shell from the equivalent deposit of Poessneck, kindly 

 forwarded to me by Baron Schauroth, have the ligament in a 

 fine state, of preservation. 



It is found in the shell-limestone only, where, sometimes asso- 

 ciated with its congener, M, costata, it is rather plentiful. 



30. Myacites elegans. King. — '^ Schlotheim's name Mya- 

 cites" (writes Mr. King, ' Perm. Mon.' p. 196) " implies that the 

 shells so called are fossil Myas : as this is not the case, the name 

 cannot stand.^^ And just above this note the same author states 

 that " the name (Allorisma) was proposed under the persuasion 

 that the cartilage-fulcra of the genus varied in position ac- 

 cording to the species ; this is now known to be an error : the 

 name is, however, still retained, notwithstanding its being a 

 misnomer." The conclusiveness of this reasoning seems to have 

 had its effect upon all careful English naturalists, for I find 

 Myacites now adopted, and Allorisma consigned to mere oblivion. 

 It still, however, lingers on the Continent, shortly to become 

 for ever extinct. 



Several authors think this species is only the PanopcEa lunu- 

 lata, Keyserling ; but I am not able to refer it to that species 

 for want of original Russian specimens to compare it with. This 

 species belongs to the genus Myacites, restricted as it is by 

 Morris and Lycett. 



The specimen which Mr. King has figured as Solemya biar- 



