THE MODIFICATION OF GENERIC CHARACTERS. 28 1 



case of Athyris proceeding onwards parallel but outside the 

 primary lamella, the second ribbon of the spiral running a 

 parallel course, but with each spiral diminishing the size of 

 the coil, and finally stopping at the apex of the spiral cone, 

 one of which is on each side of the median plane of the valve. 

 The various volutions of the coils on each side are thus called 

 primary, secondary, etc., lamellae of the spiral coil of the 

 brachidium. 



Between and uniting the primary lamellae of the two coils 

 is developed a band, variously complicated in different genera, 

 called the loop, saddle or jugum. 



In Athyris the jugum has at the centre a process extend- 

 ing upward towards the space between the crura, which is 

 called the stem of the jugum: this stem forks in the present 

 case, and the two branches (Fig. 79, b) are called arms of the 

 jugum (Fig. 79, /); they proceed on the outer side of the 

 primary lamellae almost in contact with them, forming acces- 

 sory lamella (Fig. 79, b\ In the genus Kayseria the accessory 

 lamella are continued along the face of the lamellae of the 

 spirals to form on each side a secondary or accessory spiral 

 coil. 



Indirectly connected with the modifications of the brachid- 

 ium is a calcified plate, arising from the interior walls of the 

 brachial valve along the median line, to which the jugum or its 

 processes are attached or come in contact ; this is the median 

 septum. A median septum may also be developed from the 

 corresponding position in the interior of the pedicle valve. 



Recent students of Brachiopods have found the structure 

 of the brachidium of great value in classifying the species into 

 generic groups ; and we are indebted to the work of Glass, 

 Whitfield, Bittner, Beecher, Clark, and others, that our knowl- 

 edge, systematized in the hands of the veterans Davidson 

 and Hall, is so full regarding these delicate parts of the 

 Brachiopod structure.* 



* For illustration and description of these characters of the Brachiopods the 

 student is referred to "An Introduction to the Study of the Brachiopoda," by 

 James Hall (published in the Reports of the State Geologist for 1891 and 1892 ; 

 Albany, New York) ; to the elaborate final Report on the Brachiopoda, vol. vin. 

 of the Paleontology of New York, by the same author ; to Dr. Oehlert's appendix 



