2/8 GEOLOGICAL BIOLOGY. 



in the former of the calcified supports, the brachidium, and it 

 is the sudden or rapid appearance of modifications of struc- 

 ture of this brachidium which is under discussion. 



TABLE SHOWING THE TAXONOMIC RELATIONS OF THE HELICOPEGMATA. 



Branch: Molluscoidea 



Class: <P°ly^oa 



( Brachiopoda 



Subclass: iLyopomata 



( Arthropomata 



^ J S Protremata 

 Order: ■{ 



( Telotremata 



Suborder: «j 



Rostracea 

 Helicopegmata 



i AXRYPIDy^ 



Fam.:-| Spiriferid^ 



Athyrid^ 

 [^ Ancylobrachia 



The above table is given to show the method of selection 

 of this particular group of Helicopegmata for study. All the 

 differentiation represented by the characters distinguishing 

 the particular class, subclass, order, and suborder must be sup- 

 posed to have already arisen before family characters of this 

 particular suborder could take place. 



I have adopted Dr. Beecher's ordinal classification, and take the order 

 Telotremata, which appears to be the most fully differentiated of the orders 

 of Brachiopods. The distinctive characters are found in the degree of 

 differentiation of the delthyrium, or pedicle opening, and its covering, and 

 of the brachidium or arm support. (" Pedicle opening shared by both 

 valves in nepionic stages, usually confined to one valve in later stages, and 

 becoming more or less limited hy two deltidial plates in ephibolic stages. 

 Arms supported by calcareous crura, spirals, or loops") The distinctive or- 

 dinal characters I have italicized in this definition.* 



It is within this order that we find the forms with special calcified parts 

 called deltidial plates, crura, and brachidium, either loops or spirals. The 

 subordinal distinctions are based upon the degrees and mode of elabora- 

 tion of the brachial supports. 



Rostracea is a new ordinal name proposed by Shuchert for the family 

 Rhynchonellidae of Gray, somewhat emended. It includes the genera with 

 rostrate shells, no spondylium, and the presence of crura. 



The Helicopegtnata is the group proposed by Waagen to include the 

 genera with two, calcareous, simple or double, spirally enrolled brachial 

 supports, which may or may not be attached to each other by a variously 

 constructed band or " loop." 



The third suborder is Gray's Ancylohrachia, slightly emended by 



* Beecher, " Development of the Brachiopoda," Pt. i. Am. Jour. Sci., vol. 

 XLi. p. 355, 1891, 



