BRACHIOPODA. 205 



of the valve, all traces of supporting lamellae being absent. Muscular area 

 large, flabellate and deeply excavated in the substance of the shell. Pedicle 

 impression broad, traversed medially by a longitudinal groove ; diductors ex- 

 tending for about one-half the length of the shell, their outer margins being 

 elevated ; they enclose a pair of small central adductor scars whose posterior 

 margins are raised into prominent myophores. The scars are divided by a 

 slight median septum which is continued posteriorly ; this septum being often 

 rendered very conspicuous by the growth of the shell about the apophyses 

 of the cardinal process of the opposite valve, and in extreme cases its develop- 

 ment is such that it rises above, and encloses the adductor scars, the latter being 

 excavated in its substance. 



In the brachial valve the dental sockets are long and narrow, the cardinal 

 process very large and composed of a stout, erect stem resting upon a rather 

 short median septum, and divided at its summit into two long, divergent, 

 tooth-like branches, whose upper faces extend to the interior surface of the 

 opposite valve ; hence their greatest elevation is at their anterior extremities, 

 whence they slope toward the beak of the valve, usually uniting before that 

 point ia reached. The surface of attachment of each of these apophyses is 

 medially grooved. Below them, and at the base of the central stem, arise the 

 crura, which are long, straight and slender, with expanded extremities. The 

 muscular scars are clearly defined and consist of a pair of small posterior 

 adductors, and in front of them a larger pair whose surface is radially striated, 

 the entire area being elongate-oval. Vascular impressions are occasionally 

 retained in the pedicle-valve. 



Type, Atrypa medialis, Vanuxem. Lower Helderberg group. (Delthyris 

 shaly limestone.) 



Observations. In the species of this genus the internal apophysary system 

 attains its highest development among the rhynchonelloids. Though the form 

 of the shells is invariably elongate-, or transversely subquadrate, their internal 

 characters demonstrate their close alliance to the subcuboidal shells of Uncinu- 

 LOS, and the genus prevails where the latter is most prolific, namely, in the 



