AMERICAN JOURNAL 



DESCRIPTIONS OF SOME NEW TYPES OF PALAEOZOIC 



SHELLS. 



BY F. B. MEEK. 



One of the greatest difficulties with which the student of Pa- 

 laeozoic Conchology has to contend, is the impossibility of ascer- 

 taining, in many cases, the nature of the hinge and interior of 

 lamellibranchiate types. In such cases the most usual custom, 

 where the species happen to belong to none of the known genera 

 that present well marked external distinctive characters, is to 

 refer them provisionally to such genera as they happen to re- 

 semble more or less nearly, until specimens in a condition to 

 give a clue to their true affinities can be found. In general this 

 is certainly better than to propose new groups, without having 

 seen the most important generic character. It sometimes hap- 

 pens, however, that we meet with forms presenting such strongly 

 marked external peculiarities, that we may feel well assured 

 they can safely be regarded as types of undescribed groups ; 

 and in such instances it seems to me that we are less liable to 

 err and mislead others, by at once proposing new sections for 

 the reception of such shells, than by any other disposition we 

 can make of them. 



For these reasons I have ventured to propose new groups for 

 two of the types described in this paper, without having seen 

 their hinges or internal characters. 



Genus SANGUINOLITES, McCoy.* 



Subgenus Promacrus, Meek. 



Shell thin, more or less clongate-subtrapeziform, nearly or 

 quite equivalve, either inequilateral or equilateral, the beaks 

 being nearer the anterior or posterior end, or central, according 

 to the species ; valves closed all around, and each with a well 

 defined keel, or more obtuse ridge extending from the posterior 

 side of the beaks to the posterior basal extremity ; anterior side 

 attenuated and produced ; posterior margin wider (higher) than 



*Synop. Carb. Fossils of Ireland, p. 47, 1844. 



