DIMORPHOGRAPTID^E. 347 



Affinities. — Gothog. spinosus is readily distinguished from all other British 

 Betiolites by its isolated apertural spines. 



Horizon and Localities. — Lower Ludlow Shales (zone of Monog. Nilssoni). 



Wales: Hospital Road, Builth; Irf on River, Builth. Shropshire: Middletown 

 Brook, Long Mountain; Montgomery Road, near Chirbury. 



Associates, etc. — Gothog. spinosus is a fairly common fossil in the Lower 

 Ludlow Shales (zone of Monog. Nilssoni) of the Welsh Borderland, where it is 

 found in association with Monog. Nilssoni, M. colonvs, M. bohemicus, etc. Abroad 

 it seems to occur at a somewhat lower horizon. 



Collections. — The Authors. 



Family DIMORPHOGRAPTID^J, nom. nov. 



1883. Family Heteroprionidx, Tullberg, Skaues Graptoliter II, Sver. Geol. Undersold!., Ser. C, 

 No. 53, p. 14. 



Unibiserial Graptoloidea ; proximal portion uniserial, bearing thecse of the 

 general Monograptus type; distal portion biserial, bearing theca3 of the general 

 Biplograptus type. Test continuous, membranous. 



In the Dimorphograptidge we find in combination features characteristic of the 

 Monograptidse on the one hand and of the Diplograptidas on the other. 



Proximally, the polypary in the Dimorphograptidae passes through what is 

 practically a Monograptid stage : the proximal portion is uniserial, its earlier 

 thecal originate from the sicula in the same way as in Monograptus, and show the 

 same general low inclination and slight overlap. Distally, the polypary passes into 

 a Diplograptid stage, becoming biserial, as in Dip)lograptus, the thecce showing a 

 similar higher inclination and longer overlap. 



In the family of the Dimorphograptidas we enter upon the fourth of the 

 successive types or stages in the development of the initial extremity of the 

 Graptoloidea in general. In the most ancient or Dichograptid type, the first theca, 

 after budding from the sicula, grows downward, and maintains this downward 

 direction throughout the whole of its growth. In the second or Leptograptid type 

 the first theca retains the primitive downward direction in the earlier half of its 

 growth, but in the later half it becomes approximately horizontal. In the third 

 or Diplograptid type only the initial part of the first theca retains the original 

 downward direction ; the middle portion is horizontal, and the terminal portion 

 has distinctly an upward direction of growth. In the fourth or Monograptid type 

 of development, here met with for the first time in the Dimorphograptidas, we lose 

 all trace of the primitive downward direction of the first theca, the growth of 

 which is upward from the commencement. 



