BRACHIOPODA. 347 



spire-bearing forms derive'! their brachidia from a primitive terebratuloid con- 

 dition, and this derivation has been effected by growth with accompanying 

 resorption. The progressive modification of the loop in the recent terebratellids 

 by resorption of calcareous tissue in the growth of the individual, is a well- 

 known fact which has invited the study of many investigators. In such forms 

 this modification is extreme, and is unquestionably complicated by the intimate 

 connection of the loop with the median septum of the brachial valve. With 

 the single exception of Tropidoleptus, among the paleozoic genera, there is no 

 clear evidence that the median septum has shared in, or contributed to, the 

 growth-modification of the brachial supports ; nevertheless, the outcome and 

 final result of this growth with modification in the most progressed forms of 

 Terebratella and such palaeozoic genera as Dielasma, Cryptonella, Harttina, 

 etc., is the same. 



Progressive modification of the brachial supports in both the Helicopegmata 

 and palaeozoic Ancylobrachia being now fully established, it is interesting to 

 observe that the primitive condition of the loop, as in Dielasma turgida, is one 

 of simple apposition of the two short brachial processes, at their expanded 

 anterior extremities ; having the expression of the mature loop in the genera 

 Centronella, Rensselaeria, Selenella, etc. A simple step further back would 

 afford a condition in which the brachial processes with their expanded extrem- 

 ities are not as yet united, but discrete as in the rhynchonellids. A more 

 primitive condition than that in Centronella or the centronellid stage in 

 Dielasma, could not be different from this. On the ground of these differences 

 in the conditions of the brachidium and the phyletic stages corresponding 

 thereto, it would seem fair to infer that, of the rhynchonellids, the terebratu- 

 loids and the spire-bearers, the first is the primitive stock, and the spice- 

 bearers legitimate derivations from that stock, through the terebratuloids, or 

 both of the latter derived along divergent lines from the rhynchonellids. This 

 conclusion, however coherent and consistent with the geological evidence, will 

 be found to lack stability until the data are sufficient to establish the fact that 

 the brachia themselves, and not alone their calcareous supports, have passed 

 through corresponding phases of growth and derivation. This latter question 



