BRACHIOPODA. 253 



the same time they seem, from the study of related genera, to represent a retro- 

 gression of the earlier Renssel^ria type, a consideration which is somewhat 

 fortified by the fact that Amphigenia, the earlier Renssel^ria type, appeared 

 abruptly in an enormous numerical development in the seas depositing the Upper 

 Helderberg limestones, and as abruptly disappeared with the cessation of those 

 deposits. During its existence the specific type of A. elongata scarcely varied. 



To the generic diagnosis above given the following amplification in some 

 structural details may be added : 



The prevailing form of the mature shell is elongate-ovoid, high shouldered, 

 broadest behind and sloping to a narrow and graceful curve anteriorly. This 

 is quite the same character of outline as that occurring in Rensselaria ovoides. 

 In young shells this prominence of the posterior portions is not attained, and 

 they have the sloping shoulders and general expression of the mature shells of 

 Newderria, a later and immediately succeeding genus. The obtuse median fold 

 on both valves at maturity is also less clearly defined in youth. The surface 

 is generally smooth with irregularly distant festoons or wrinkles of growth, 

 and distinct radiating striae may usually be seen, especially in young indi- 

 viduals. 



The substance of the shell is impunctate on the surface. Beneath the epidermal 

 layer is another that is highly punctated ; within this, and apparently forming 

 the innermost lamina, is a third layer whose surface is minutely wrinkled, and 

 is very suggestive of the " Runzelschicht " of the ammonoid cephalopods. 



In the pedicle-valve the spondylium is short and is distinctly formed by the 

 union of the dental lamellae with a medium septum. In Renssel^eria these 

 lamellae are highly developed, but are usually appressed against the lateral walls 

 of the valve, being free only at their anterior margins, while between them lies 

 the well-defined and longitudinally-divided muscular area. It is a natural . 

 inference, therefore, that the muscles of Amphigenia were implanted on the spon- 

 dylium, and there is no evidence of scars elsewhere in this valve. As in Rens- 

 8EL.s;eia, however, the vascular sinuses are visible on the post-lateral surfaces. 



In the brachial valve the hinge-plate is not always perforated by the visceral 

 foramen, though it probably has been at some time in the history of the indi- 



