NATURAL SCIENCES OF PHILADELPHIA. i 



especially of the type of Camarium, Hall, an arching lamella arises from the 

 basal portion of each dental plate, and the two unite in the mesial line of the 

 valve, forming a structure which Prof. King, before the separation of this genus, 

 had styled the shoe-lifter process, arched in front, and attached to the bottom 

 of the valve behind. In Spirifera granulifera, Hall, a horizontal transverse 

 plate stretches across the middle of the beak of the ventral valve, connecting 

 the dental lamellae where nearest approximated by their inward curvatures, a 

 stucture which probably represents the pseudo-deltidium of certain Spiriferse, 

 but not of Cyrtia. Beneath this plate, the ventral medium septum assumes the 

 form of a tapering cone, resting with its base filling the cavity and having the 

 anterior part of the upper side marked by a longitudinal groove or slit, while 

 the posterior part sends up a small vertical plate to the transverse plate just 

 mentioned. In Syringothyris, the transverse plate equally connects the dental 

 lamellae where most approximated, and is somewhat arched upward, as in Me- 

 rista, but it does not join the bottom of the valve as in that genus, nor is it 

 connected with the median septum as in Spirifera granulifera. Nevertheless it 

 would seem that the three structures are modifications of the same elements. 



But what is the element thus modified ? Prof. King suggested that the shoe-lifter 

 process of Cleiothyris concentrka is a modified form of the ventral median plate; 

 but the wide separation of its points of origin from the normal position of thisplate 

 seems incompatible with such a conclusion ; while in Syringothyris and Spiriferse 

 granulifera the median plate exists independently of the apparent homologue of 

 the shoe-lifter. Mr. Billings, whose observations are generally marked by ex- 

 treme sagacity, regards the shoe-lifter "as an abnormal form of the pseudo- 

 deltidium that occurs in some Spirifers." This is the relationship pointed out 

 above ; and there seem to exist good morphological reasons for regarding the 

 fistuliferous arching plate of Syringothyris as a modified pseudo-deltidium. But 

 to what does the latter structure appertain ? In Merista, Syringothyris and cer- 

 tain Spiriferse its relation to the dental plates suggests that it may be an out- 

 growth of those parts. The dental plates are amongst the most heteromorphous 

 structures of the ventral valve. From a normal erect position, they become ap- 

 proximated along the ventral margins in many Spiriferse and other genera, while 

 in Pentamerus, Orthisina and Camarophoria this approximation results in com- 

 plete union, and in Leptsena in the formation of the saucer-shaped process of 

 the ventral valve. They also vary excessively in longitudinal development. 

 In many Spiriferse, moreover, there is an evident indication of a longitudinal fold- 

 ing of the dental plates, producing on one side or the other a longitudinal lami- 

 nar process, which, under an extraordinary development, may coalesce with 

 some neighboring part. While, therefore, the shoe-lifter process of Merista, and 

 still more the fistuliferous diaphragm of Syringothyris, may be but modifications 

 of the false inner deltidium of Spirifera granulifera, the three structures acci- 

 dental among Palliobrancbs may be but mere outgrowths of the essential and 

 typical parts known as dental plates. 



The geological range of the above generic type is, as far as known, only from 

 the base of the yellow sandstones at Burlington, Iowa, to the Keokuk limestone. 

 The species from the latter horizon cannot at present be characterized. There 

 are reasons for believing that Spirifer extenuatus, Hall, from the yellow sand- 

 stones at Burlington, will yet be found to possess the same peculiarities, if it is 

 not a variety of one of the following species. 



Syeingothyris typa, n. sp. Shell large, thin, externally destitute of radiating 

 ribs, or showing them but faintly. Ventral valve with a broad, undefined, rather 

 shallow sinus ; beak extremely elevated ; slope thence nearly straight to all parts 

 of the margin ; area very large, triangular, flat, forming an angle of about 30 

 with a line along the bottom of the sinus, and perforated by a rather broad tri- 

 angular fissure. Dental plates diverging at an angle of 30, continued nearly 

 to the apex of the beak, and extending anteriorly beyond the middle of the 

 valve. Attachments of the myary system unknown. 



1863.] 



