Permian Productinae and Strophalosiinae of W.A. 
.) 
The area in both these genera possesses a distinct pseudo-deltidium and 
in Strophalosia a pseudo-chilidinm in the brachial valve. The terms pseudo- 
deltidium and pseudo-ehilidium have not the significance usually given them 
in the description of embryonic brachiopods. The division of the area into 
two lateral parts by the pseudo-deltidium is undoubtedly due to the inter- 
ruption of the deposit of CaCOg along the cardinal margin by the projecting 
ridges, the pseudo-deltidium being that part of the shell filling the space 
between these ridges. At the places of formation of the ridges there is excess 
CaCOg deposit and this projects slightly above and below the level of the 
area, as Frederick's ‘^delthyrial ridges” above and where the downward 
projecting ridges are more pronounced they take on the function of and are, 
teeth. In Strophalosia the angle of divergence of the sides of the pseudo- 
deltidium gives some indication of the angle of divergence of the roots of 
the teeth though not necessarily of the teeth themselves as these may extend 
laterally from their roots. That portion of the hinge-area, called, in the 
brachial valve, the pseudo-ehilidium is the posterior root of the cardinal 
process. The narrow grooves limiting its extent are the internal edges of 
the sockets. In A.ulosteges, which has no brachial area, the posterior root of 
the cardinal process is produced beyond the hinge-line as a flat triangular 
plate, in the plane of the brachial valve, which partly fills the delthyriuin. 
The pseudo-deltidium of Auhsteges may bear spines. 
Internal Features. 
Brachial Valve. — However the external appearance of the brachial valve 
may vary on the inside it always has a flat visceral disc with possibly down- 
turned margins. A medium septum arising at the base of the cardinal process 
divides the shell into two halves. The septum varies in length from one-half 
to two-thirds of the length of the visceral disc. 
Muscle Impressions. — The adductor muscle impressions lie posterior to 
the middle of the valve on either side of the median septum. They are den- 
dritic in Aulosteges and Productus s.l. but non-dendritic in Strophalosia. 
The presence of dendritic muscle impressions in Aulosteges and Productus 
s.l. is a reflection of the poorly developed articular apparatus in these two 
genera. Since the brachial valve is pivoted loosely on the cardinal process 
it is capable of movement not only in a plane perpendicular to the surface 
of the valve but also from side to side. This lateral skewing movement would 
cause a differential movement within the muscle itself, the fibres nearer the 
centre being less extended than the excentric fibres. A divided muscle would, 
therefore, be mechanically superior. In examining a dendritic muscle it will 
be noted that the bunches of fibres have an approximately radial arrange- 
ment with the radii on the inside much shorter than those towards the lateral 
margin. As the central muscles have a shorter pull this arrangment is what 
would be expected. 
In Strophalosia the muscle scars are non-dendritic but, except in 
Wgndham.ia, are divided into posterior and anterior parts. The anterior 
adductor is elongated in the longitudinal axis of the shell and lies very close 
to the median septum while the posterior jiortion is elongated laterally, lies 
above and close to the anterior adductor, but extends beyond it. The brachial 
impressions arise at the lateral terminations of the posterior adductor im- 
pressions. 
