BRACHIOPODA. 67 



consists of two lateral branches, broad at their origin, inclined backward, and 

 uniting to form a stem which bears a short bifurcation at its extremity. 



The muscular area is elongate-ovate and more or less distinctly separated 

 into anterior and jjosterior scars. Surface of the valves smooth or with fine 

 concentric growth-striae. Shell-substance fibrous, impunctate. 



Type, Merisiella Maria, Hall. Niagara group. 



Observations. The name Meristina, proposed in Volume IV of the Palaeon- 

 tology of New York (p. 'j!99), was introduced for the purpose of distinguishing 

 from Mekistella a species, M. Maria, Hall, which possesses strong meristelloid 

 characters but lacks the peculiar loop of that genus. Though the loop was im- 

 perfectly represented in the figure accompanying the first use of the name, it 

 nevertheless constituted then, as it still does, the single important difference of 

 the species from Meristella. The precise character of this loop was fully 

 determined subsequently (as described and illustrated in the present work), by 

 the Rev. Norman Glass, Irom specimens obtained at the celebrated locality, 

 Waldron, Indiana,* and described by Dr. Davidson in 188'2.f Mr. Glass found 

 a loop of like structure in the English ( Wenlock) examples of the Atrypa tumida, 

 Dalman. In the place cited Dr. Davidson expresses his conviction of the 

 identity of the American species M. Maria with Atrypa tumida, and as the form 

 of the loop then determined was new, he proposed to distinguish these fossils 

 by the generic name Whitfieldia. 



It is to be regretted that the laws of nomenclature do not permit the 

 admission of this name. Whether or not Dalman's species and the American 

 M. Maria be conspecific, :]: they are at all events congeneric, and belong to the 

 much earlier genus, Meristina. That this genus was imperfectly described 



* Though this species is a rare fossil in the Niagara fauna of New York, it is very abundant at Waldron 



and far fi'om infi-equent in the Niagara dolomites of northern Illinois and southei-n Wisconsin. 



t Silurian Supplement, p. 108, pi. v, fig'. 6. 



I We do not follow Mr. Davidson in reg.irding these forms identical. They jiresent differences which 

 though slight, are positive and permanent variations of the same type of structure. A series of Gotland 

 specimens of Atrypa tumida, obtained from, and identified by Dr. Lindstro.m, and submitted for our exami- 

 nation by Mr. Charles ScHncHERT, shows that there are two readily apparent variations in the foi-ms refer- 

 red to the Swedish species, and it is an interesting fact that these are from different localities. One of these 

 forms (fi'om Westeigarn) is of small size, strikingly subpentagonal outline, with high, strongly arched and 

 nari'ow umbo on the pedicle-valve, the greatest diameter of the shell being ^in front of the middle ; while 



