BRACHIOPODA. \m 



examination, is as distinctly corneous as in Linuula, anil from the manner of 

 exfoliation seen in the shells, is evidentl}- composed of successive lamintc. 

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



The species of Linnaussonia thus far knowu are all from primordial faunas ; 

 L. transversa, Hartt, and L. niisera, killings (Matthews), from the St. John };roup; 

 L. Taconica, Walcott, from the Taconic limestones iu Washin,^'tou county, N. 

 Y. ; L. preUosa, Billings, from the Quebec group, ;it fjittle 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 Obohlla maculata, Hicks, from St. David's, will 

 probably prove i-eferable to this genus. 



Genus M E S T 11 E T A , Kutorca. 1848. 



I'l.ATE IV, I'lG. 21. 



1S48. tS'ip/ioiKitreta (Mesotreta). Kiitohi;a. Ut-bei- die Br;icliioiHHloii-l''!imilie cl<;r Siiilioiu)treta;a' ; Ver- 

 hanill. del- I'lis-s.-kais. mineral. Gt'.<iellsdi. zii St. Petersburg. .Iiihrg-. 1847, p. 271, 

 pi. vii, fig. 4 a, I/, c. 



1S71. Siphuuotreta, QvEtiSTKDT. Petrefakteiikmnle Deutselilnncls; ISrachioiio.leii, )>. i;74, pi. 61. fif:. 33. 



Diagnosis. Shell patellilbnu, depressed-conical; apex central, perlbrated by 

 a foramen ; no area or (-inlinid tlattening. Surlace 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 nuist 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. 



