BRACHIOPODA. 109 



examination, is as distinctly corneous as in Lingula, and from the manner of 

 exfoliation seen in the shells, is evidently composed of successive laminse. 

 The more intimate structure of the shell-substance is yet to be ascertained. 



The species of Linnarssonia thus far known are all from primordial faunas ; 

 L. transversa, Hartt, and L. ?nisera, Billings (Matthews), from the St. John group; 

 L. Taconica, Walcott, from the Taconic limestones in Washington county, N. 

 Y. ; L. preiiosa, Billings, from the Quebec group, at Little Metis and elsewhere 

 in Canada; L. sagittalis, Salter, from the Menevian of St. David's and else- 

 where in Wales, in the Paradoxides beds at Bornholm, Sweden, and at Mt. 

 Stephen, British Columbia. The Obolella maculata. Hicks, from St. David's, will 

 probably prove referable to this genus. 



Genus MESOTRETA, Kutorga. 1848. 



PLXTE IV, I'IG. 21. 



1848. Siphonotreta (Mesotreta), Kutokga. Ueber die Brachiopoden-Familie der SiphonotretaeEe ; Ver- 

 handl. dei- russ.-kais. mineral. Gesellsch. zu St. Petersburg-. Jahrg-. 1847, p. 271, 

 pi. vii, fig. 4 a, h, e. 



1871. Siphonotreta, Qdenstbdt. Petrefaktenkiinde Deutschlamls ; Bracliiopodeii, p. B74, pi. 61, fig. 33. 



Diagnosis. Shell patelliforra, depressed-conical ; apex central, perforated by 

 a foramen ; no area or cardinal flattening. Surface ornamented by concentric 

 growth-lines bearing spines. Pedicle-valve only known. 



Type, Siphonotreta tentorium, Kutorga. 



We have elsewhere referred to the fact that this name was provisionally 

 proposed for a species representing one extreme of variation in the position of 

 the pedicle-aperture, allowed under Kutorga' s diagnosis of the genus Siphono- 

 treta. Under the original limitations of the latter genus, this form must be 

 excluded as not being congeneric with S. unguiculata. When Mesotreta is bet- 

 ter known, it may be found to represent an important connecting link between 

 Acrotreta and Schizambon. 



