GRAPTOLITES OF NEW \ORK, PART 2 389 



as long as only shale material is available. Lapworth further describes the 

 Scottish and Irish representatives ot the species as possessing lateral spurs 

 at a short distance from the sicula \_see text fig. 9]. These suggest 

 homology to the rows oi lateral spines of Glossograptus and it is there- 

 fore logical that this author points out the probable near relationship 

 of Orthograptus quadrimucronatus to Glossograptus. It might 

 here also be remarked that the broad smooth flat lateral faces of <| 11 ad r i- 

 m 11 c r o n a t us find their exact counterpart in those of Glossograptus. 



On the whole, it seems that Freeh's proceeding in separating- the spe- 

 cies of Diplograptus with spines from those without spines is correct and 

 that the spinose species will all, in time, be found to have a similar structure 

 as G . q u a d r i m u c r o n a t u s and be removed from the genus altogether. 



The beautiful material from the Rural cemetery has permitted in many 

 specimens a very clear observation of the youngest stages of the rhabdo- 

 some. In these [see pi. 27, fig. 8, 9] the sicula is seen to be provided with 

 distinct growth-lines, the first theca to originate very near the distal end of 

 the sicula, to grow forward as far as the apertural margin of the sicula and 

 then to bend outward and finally backward. This genus shows hence the 

 same remarkable reverse of the first theca, as Diplograptus and Climaco- 

 graptus. Also the mode of budding and arrangement of the succeeding 

 thecae are as far my observations go, the same as in Diplograptus. The first 

 and second thecae bear instead of the two lateral spines of the apertures seen 

 in the later thecae, but one frontal spine each. These two spines are, as in 

 the Rural cemetery material, liable to grow out to immense size and to 

 form with the equally growing virgella of the sicula a strong tridentate organ 

 of defense [see text fig. 341]. Sometimes also the virgella fails to grow out 

 and only the two thecal spurs appear on the sicular extremity of the rhabdo- 

 some, as in the specimen reproduced in text figure 340. 



Hall remarks upon the great number and difference of the aspects 

 offered by this species as follows: 



The species under consideration, in its various aspects, illustrates more 

 fully than any other which we have seen the effects of pressure in different 

 directions. 



