110 PALEONTOLOGY OF NEW YORK. 



The external surface is radially plicate, the plications being simple. In young 

 shells there is a median sinus on both valves, but as growth advances, that of 

 the brachial valve develops into a low fold. Both fold and sinus bear a num- 

 ber of small, intercalary plications, much finer than those adjoining on each 

 side. Shell-substance rather sparsely punctate. 



Type, Waldheimia formosa. Hall. Lower Helderberg group. 



Observations. This generic division was separated from Retzia at a time when 

 certain Carboniferous species with subalate cardinal extremities were regarded 

 as typical of that genus. Subsequently these later species were found to differ 

 from the strict Devonian type of Retzia, and a distinctive name, Eumetria, 

 was proposed for them in 1864. In consequence of this, and while the typical 

 Retzia, R. Adrieni, was less accurately understood than now, the term Rhyn- 

 CHOSPiRA fell into quite general disuse, its species being commonly referred to 

 the old genus, Retzia. Evidence has already been given demonstrating the 

 peculiar distinctive value of Retzia as based on its typical species, and though 

 there is close external resemblance between the Devonian species, R. Adrieni, 

 and the earlier typical forms of the genus Rhynchospira, there is no longer 

 any justification for associating the two in one division. 



There are some important features which these two groups possess in com- 

 mon, and which, indeed, may be shared to a greater or less degree by all retzioid 

 genera. In exterior structure, the finer division of the median plications, the 

 smooth, gradually sloping umbo-lateral areas, occur in both Retzia and Rhyncho- 

 spira. The coalescence of the deltidial plates is a feature occurring throughout 

 the retzioid genera, though the union is perhaps more completely effected and 

 subject to less variation in the Carboniferous forms. Retzia and Rhynchospira 

 have a similar structure in tlie hinge-plates ; and a character which occurs 

 persistently in these genera and also in Trematospira and Parazyga, is the 

 narrowing of the lateral branches of the loop just above their points of origin 

 on the primary lamellae. Retzia, however, possesses a split deltidial tube in 

 the umbonal cavity, which is wanting in Rhynchospira, and also a bifurcate 

 termination of the stem of the loop. 



