Vol. 56.] 



SI LU EI AN SEQUENCE OF EH A YA DEE. 



133 



virgula. Distally only 24 to 26 occur within the length of 1 inch, 

 each theca being about 5 times as long as broad, and inclined 

 at an angle of about 25°. They present the appearance of cylindrical 

 tubes overlapping one another . 



Fig. 21. — Diplograptus magnus, 

 sp. nov. (a 6f b, nat. size ; 

 c tf-d, X5). 

 a b 



from two-thirds to three-fourths 

 of their length ; they usually 

 contract at their junction with 

 the apertures of the preceding 

 fchecae, and again expand towards 

 their own apertures. The form 

 of the free portions of the thecae 

 varies with the amount of com- 

 pression to which the graptolite 

 has been subjected, sometimes 

 resembling the Climaeograptm- 

 type, with external margins 

 nearly vertical and proximal 

 edges horizontal, or appearing 

 in the form of straight in- 

 clined tubes with parallel 

 margins and eoncave apertures 

 perpendicular to the direction 

 of the theea3. The lines of 

 growth are generally well ex- 

 hibited under a low microscopic 

 power. 



A straight median line is 

 always distinct in obverse aspect, 

 and in the reverse view the im- 

 pressed furrow runs from end to end of the rhabdosoma. There 

 appears to be no prolongation of the virgula distally, or of the virgella 

 at the proximal end. 



Proximal end.- 

 Its apex reaches the level of the second aperture on the primordial 

 series, and appears to be completely visible in obverse aspect. It is 

 tree on its dorsal side for about one-third of its length, and in 

 reverse aspect nearly the whole of this length is exposed. The first 

 theca has apparently grown downward, and extended a short dis- 

 tance below the proximal end of the sicula, with a total length of 

 about two-thirds of the latter. 



This species is most closely allied to Diplograptus truncatus, Lapw. 

 and D. palmeus, Barr. From the former it may be easily sepa- 

 rated by 



(1) The size of the mature rhabdosoma ; 



(2) The absence of spines, and the general characters of the proximal end, 



(3) The presence of a visible virgula ; and 



(4) The form and inclination of the thecas. 



From Diplograptus palmeus it is distinguished by 



(1) The size and form of the mature rhabdosoma ; and 



(2) The characters of the proximal end. 



x 0. 



a = Obverse aspect, restored. 



b — Reverse aspect of an abnormally 



contracted specimen, drawn from 



the actual specimen. 

 c=Proxiinal end : obverse, restored. 

 d= Proximal end : reverse, drawn from 



the actual specimen. 



-The sicula has a constant length of yL inch, 



