52 PALAEONTOLOGY OF NEW YORK. 



valve is the great post-median depression, which was considered by Mr. Fokd 

 as representing a pit for the reception of the pedicle, but the necessity for so 

 large an organ in so small and light a shell, as this would imply, does not appear, 

 and we have regarded it as the correlative of the central muscular scar in the 

 opposite valve. From the anterior end of this scar are two narrow diverging 

 furrows, the function of which is not understood, and the analogues of which 

 have not been observed in any of the oboloid or indeed among the inarticu- 

 late brachiopods in general, save in the species Monomer ella Greenii, where they 

 attain the same relative development as in Elkania. It is possible that these 

 furrows are specialized sinuses leading from the genitalia to the pallial vascular 

 system, and performed the function of oviducts; their juxtaposition to the pallial 

 trunks indicating some such office. Of still further importance in Elkania is 

 the narrow furrow in each valve passing around the muscular area just within 

 the cardinal margin, tei'minating laterally in a more of less well defined scar ; 

 in other words the first appearance, in the oboloid genera, of a distinctly de- 

 veloped crescent. As to their appearance in time, Obolus, Elkania and Ding- 

 bolus are successive. 



If we have, thus, correctly suggested the source and development of the 

 platform in the Trimerellids, we are confronted with the interesting phe- 

 nomenon of a similar resultant attained along different lines of development. 

 This may be expressed by the following diagram : 



^^ Oboliis Elkania ^^ 



1 <(' (LaJchmina) ~^> Ti-imereUa. 



^Lingulella -^ /^„^MonomereIla Rhinobolus. 



^^^ Lingula Lingulops Lingulasma'^ 



It is probable that the progenitor of Obolus will be found in Obolella, or some 

 of the closely allied genera, and our present knowledge of Ltngulella is such 

 that, as elsewhere observed, it must be placed in juxtaposition to Obolella, 

 while evincing fi deviation toward Lingula. The two are, in all probability, 

 but a few removes from their common ancestor, which may be looked for in 

 the earliest primordial faunas, unless its existence was limited to obliterated 



