1876.] 375 [Hyatt. 



Steph. dimorplium constitutes a series by itself, or rather it might be 

 said begins one of which it is the only known member. The evidence 

 afforded by the earlier stages of growth indicates a close affinity with 

 Brocchii, since the shell evidently continued to increase the size of 

 the living chamber until the adult period. At this stage it began to 

 exhibit very markedly the contraction previously described. The 

 presence of the furrows also shows that this living chamber was 

 never absorbed to any great extent; a very remarkable difference 

 when we consider, that if the furrows were absent many English 

 specimens would be inseparable from microstomum in Dorsetshire, and 

 others found in Calvados, ^indistinguishable from Brongniartii of the 

 same locality, and that both of these species habitually absorb the 

 living chamber after every arrest of growth. The mouth outlines 

 agree with those of the preceding series. 



The next and last series with which we have to deal is also the 

 most extraordinary. 



Step by step, in spite of preconceived notions, the evidence has 

 forced me to refer the whole of these series, which spring from con- 

 traction, to Steph. subcoronatum as the parent form, and this is the 

 case here also. 



The connection of Steph. Braikenridgii with this species is equally 

 plain, although the large lappets are so distinct that an independ- 

 ent origin might have been reasonably anticipated. The resem- 

 blances of the young of Steph. Braikenridgii, to the young and adult 

 of Steph. subcoronatum are too plain to admit of much doubt, and 

 it is probable that the blank which still exists will be filled, as it 

 has been in the genetic history of Amm. fuscus by Quenstedt, by the 

 discovery of intermediate forms having the mouth lappets as a varia- 

 ble or simply adult characteristic. The young of Steph. Braikenridgii 

 resemble the adults of subcoronatum, with the exception of the con- 

 traction of the living chamber. This takes place in young speci- 

 mens, however, much more slightly when an inch or half an inch in 

 diameter than it does in the full grown, and at no period does it equal 

 the distortion common in the next member of the series, Steph. Sauzei. 

 The mouths of both species not only have the lappets, but these are 

 peculiar in arising from the abdomen and spreading out abdominally 

 instead of laterally, in correllation with the abdominal flattening of 

 the outline, which gives the shells a totally distinct aspect from those 

 of any other series. Steph. Sauzei accelerates the inheritance of the 

 subcoronatum form so much that it is difficult to recognize the affinity 



