GENERA OF THE SUBORDER ORTHOIDEA 



153 



where its position is equally anomalous with the one 

 described above. The presence of endopunctas at once 

 rules the genus out of the Strophomenacea (Kozlowski 

 having shown that the shell substance of the stropho- 

 menoids is impunctate), and there are therefore only 

 two possible places to put it, either in the Dalmanellacea 

 or in the Terebratulacea. It appears to us now, after 

 a long study of the orthoid genera, that the best — at 

 any rate, the least anomalous — position is in the Dal- 

 manellacea. We would place it here because of the 

 punctate shell, the open delthyrium, and the general 

 orthoid nature of the brachial processes, which was 

 suggested by Hall long ago. The endopunct.-c of 

 Trop'uioUptus are very similar to those of Dalmanella 

 and allies, but are not so close to those of the Tere- 

 bratulacea in pattern, the latter being much more 

 dense. 



The structure of Tropidoleftus accords well with 

 that of the dalmaneUoids in having an open delthyrium, 

 a wide hinge-line, and an interarea on both valves. 

 In the dorsal valve there is a small chilidium such as 

 one finds in Heterorthis and other punctate genera. 



The ventral musculature is suggestive of the orthids, 

 as Hall pointed out. In the dorsal valve the cardinal 

 process is bilobed but is not of the type common in 

 Strofhomena and allies, in which the lobes are usually 

 isolated. The teeth and sockets are grooved much as 

 in Parmorthis among the dalmaneUoids. The so called 

 loop is of course the most difficult structure to account 

 for. This in itself, however, does not have the appear- 

 ance of the true terebratelloid loop that grows out from 

 the septum and unites with the descending lamellae 

 of the crura; rather these processes are nothing other 

 than very long crura as in some Rhynchonellas, but 

 instead of remaining free they appear to unite with the 

 median septum. We hold that this type of arm sup- 

 port could have developed many times, instead of but 

 once. In conclusion, then, we believe that Tropi- 

 doleptus is a highly specialized and terminal dalman- 

 elloid of short geological life." 



At present it is difficult to say definitely out of what 

 dalmaneUoids Tropidoleptus may have come, but we 

 suggest that it may have been out of the Parmorthis 

 stock. 



