698 NEW YOKK STATE MUSEUM 



There is, to state this diiBciilt and confused relation of the various names 

 more concisely, no doubt in our mind that D. caduceus Salter should 

 have preference to Nicholson's later and better known name, D. gibber- 

 u 1 u 8 ; that Salter's name can not be applied to T e t r a g r a [> t u s b i g s b y i 

 Hall, which name in its turn has to yield to the older name Tetragraptus 

 similis Hall. 



Didymograptus caduceus Salter nanus mut. nov. 



Plate 15, figures 8, 9 



A small form, which I had cited in Museum bulletin 52, 1902, loc. cit. as 

 D . g i b b e r u 1 u s var. n a n u s , but which more correctly should be desig- 

 nated as D . caduceus mut. nanus, occurs quite frequently in the last 

 horizon (graptolite bed 7, zone with Diplograptus den tat us). It 

 y differs from D . caduceus in its constantly smaller 



/ size (the largest specimen observed having a length of 



I vy but 5 mm), its wider proximal part (2.3 to 2.5 mm), the 



'^^^^^^^T abrupt narrowing of the branches [fig.8], the smaller 



""j^^^^^^K size and closer arrangement of the thecae (14 to 18 in 

 ^^^^^^^ 10 nim), the distal convergence of the branches and the 



^^^ spinous pi'ocesses of the lower ends of the apertural 



Fig. 90 Dldymograp- margins, 

 tus caduceus Salter , i i • t , , ,• e 



nanus mut. nov. Deep 1 hese characters clearly mdicate a concentration ot 



kill. x7 •' 



the entire development of the I'habdosomes into smaller 

 space and shorter time, denoting the paracmic condition of the mutation. 

 This concentration finds its most pregnant expression in the specimens, of 

 which figure 90 is an example. In this the distal points of the two branches 

 point toward each other and have approached so closely that the further 

 gemmation of thecae appears impossible, and a further growth of the branches 

 will consist only in the prolongation of the last thecae, Avhich will produce 

 the rapidly tapering branches of figure 8. The formation of long spines is a 

 secondary character, acquired in this stage. 



