928 Report or THE State GEOLOGIST. 
that the spire-bearing forms have derived their brachidia from a 
primitive terebratuloid condition, and this derivation has been 
effected by growth with accompanying resorption. The pro- 
gressive 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 investi- 
gators. In such forms this modification is extreme and is 
unquestionably complicated by the intimate connexion of the 
loop with the median septum of the brachial valve. Among the 
paleozoic genera there is, with the single exception of TRo- 
PIDOLEPTUS, no clear evidence that the median septum has shared 
in, or contributed to the growth-modifications of the brachial 
supports; nevertheless, the outcome and final result of this 
growth with modification in the most progressed forms of 
TrREBRATELLA and such paleozoic genera as Dretasma, Crypro- 
NELLA, Harrrrna, etc., is the same. 
Progressive modification of the brachial supports in both the 
Hexicorremata and paleozoic AnoyLopracura 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 CenrroNnELLA, RensseLeriA, SELENELTA, etc. A simple 
step further back would afford a condition in which the brachial 
processes with their expanded extremities are not as yet united 
but discrete as in the rhynchonellids. A more primitive condition 
than that in CenTtroneta or the centronellid stage in Dizrasma, 
could not be different from this. On the ground of these differ- 
ences in the conditions of the brachidium and the phyletic 
stages corresponding thereto, it would seem fair to infer that 
of the rhynchonellids, the terebratuloids and the spire-bearers, 
the first is the primitive stock, and the spire-bearers legiti- 
mate derivatives of that stock, through the terebratuloids, or 
both of the latter derived along divergent lines from the 
rhychonellids. This conclusion, however coherent and con- 
sistent 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 sup- 
ports, have passed through corresponding phases of growth and 
180 
