GRAPTOLITES OF NEW YORK, PART 2 453 



pointed out by Freeh would place the form close to M . priodon van 

 rimatus Perner, which however, has shorter apertural tubes. A compar- 

 ison of Perner's exact camera drawing of the earliest portion of the rhab- 

 dosome [see his pt 3, sect, b, p. 2, fig. 1] of a specimen in relief, with a like 

 drawing of a pyritized specimen of our material [see text fig. 428] brings out 

 at once the fact of the different rate of growth of that portion ; that of M . 

 p r i o d o n attaining very rapidly its full width and that of M . c 1 i n t o n- 

 e n s i s growing very gradually. 



The earh' thecae possess distinct apertural spines [see tig. 428], a feature 

 not observed in any of the varieties and mutations of M . priodon with 

 the exception of the variety flemingi (Lapworth) Freeh. To the latter, 

 which, however, is considered by Lapworth as a distinct species, the imma- 

 ture thecae of our form would seem to approach most in their apertural 

 characters while the adult thecae are those of a priodo n. On account of 

 the differences in the early rhabdosome, here mentioned, we have preferred 

 to recognize M . clintonensisasa distinct species of the h a 1 1 i or 

 p r i o d o n group. 



The two sections through pyritized adult thecae here introduced 

 I fig. 429, 430] are best suited to show the true form of the thecae and a 

 comparison of the figures with the section through a typical M . prio- 

 don given by Perner [see text fig. 425] demonstrates the difference in their 

 thecal forms. 



While there exists a distinct differentiation of the earlier and later 

 thecae of the rhabdosome, it is by no means so marked as Hall's figures 

 [loc. cit. pi. B, fig. 1, 2] and description [1859, P- 2 9] would suggest. Hall 

 states that in the mature thecae the aperture is directed downward and in 

 the less mature upward, the latter possessing an angular aperture. The 

 aperture is however directed downward in all thecae and the difference is 

 only one of degree of development of the apertural tube, the latter gaining 

 gradually in length with the growth of the rhabdosome. The triangular 

 aspect of the early thecae in the pyritized specimens is largely due to the 

 failure of the thin tubular distal portion of the thecae to become pyritized. 



