C. E. Beecher — Koninckina and related Genera. 'bYl 



Before discussing the position held by the genus Davidsonia, 

 attention is called to the development of the deltidium and 

 pseudo-deltidium, or pedicle-sheath. It has been shown by J. 

 M. Clark and the writer (loc. cit.), that all the species, so far 

 as examined, possessing a true deltidium in the adult state, 

 show that it was gradually developed in early stages of growth, 

 by concrescence along the lateral margins of an open triangular 

 area. Also that all species furnished with a pedicle-sheath 

 have it fully developed in the earliest growth-stages which 

 have been observed for these species, and the subsequent 

 growth of the individual does not materially alter its general 

 characters, except that it is sometimes retrogressive, the parts 

 becoming atrophied or functionally obsolete. A feature of 

 such importance, and so intimately connected with the em- 

 bryonal growth of the shell, must be given considerable sig- 

 nificance in discussing the various genera in which it is present 

 or absent. 



Davidsonia, has always been described as having the area 

 covered by a pseudo-deltidium, and this is the first objection to 

 the grouping of the genus with Koninckina and Amphiclina. 

 So far as known, the arms were not supported by a calcareous 

 ribbon, but were fleshy and perhaps movable, as in Rhyn- 

 chonella, and furthermore, no true spire-bearing form has yet 

 been shown to have a pedicle-sheath. With our present in- 

 formation, exception must be made for Tliecospira, but farther 

 investigation in that genus may result in proving it to have 

 deltidial plates, as has been accomplished in Spirifer and Spir- 

 iferina* ■ although these genera have commonly been de- 

 scribed as having a pseudo-deltidium, as in strophomenoid 

 shells. The muscular system of Davidsonia corresponds more 

 closely with the Strophomenidse and Productidse than with 

 spire-bearing genera, and in view of these facts collectively we 

 reject Davidsonia from the Koninckininse. 



The genus Koninckella should remain as placed, as all its 

 characters harmonize with those included in our present un- 

 derstanding of the leading genus of the group. Formerly the 

 presence of a well-developed hinge area and pedicle perfora- 

 tion in this genus did not agree with the characters ascribed 

 to Konincktna, but this ohjection is now removed. 



Of the three remaining genera little of a positive nature 

 can be stated without further examination of material. The 

 descriptions and figures of Anoplotheca which have been given 

 are not sufficient to exclude the genus nor to show that it 

 actually belongs here. C'aslospira, however, has been clearly 

 shown by Davidson to be closely related to Atrypa and Zygo- 



* Development of some Silurian Brachiopoda, pp. 78-89. 



