DIPLOGRAPTUS. 



265 



Figs. 181 a — c. — Mcsograpius modestus, 

 var. parvulus (H. Lajjw.). 



i 



a Glimacograptus, and it was this character which naturally led to its being referred 

 to the genus Glimacograptus. The free ventral margin of the theca, however, 



is always slightly inclined, and both in this respect 

 and in others the thecse of var. parvulus agree with 

 those typical of the sub-group Mesograptus. 



All the characters of this diminutive form agree 

 precisely with those of the proximal part of the exam- 

 ples of Mesog. modestus, with which it is associated 

 (comp. Figs. 180 c and 181c). In our opinion, there- 

 fore, it can at the most be regarded only as a dwarf 

 variety of that species, one in which the growth of 

 the polypary has been arrested at a very early stage. 

 The distinction of var. parvulus as a separate 

 variety finds support on stratigraphical grounds ; 

 for it occurs in abundance in the lowest beds only 

 of the Llandovery in Wales and elsewhere, and 

 is not known to survive into the zone of Orthog. 

 vesiculosus, where Mesog. modestus itself is most 

 abundant. 



Horizon and Localities. — Lower Llandovery, Birkhill Shales, Lower Gwastaden 

 Series (zone of Oephalog. acuminatus). 



Wales : Rhayader ; Fuches-gau Farm, near Pont Erwyd, Cardiganshire. S. 

 Scotland : Dobb's Linn. 



Associates. — Var. parvulus occurs at the base of the Gephalog. acuminatus zone 

 in the Pont Erwyd district associated with Mesog. modestus and Glyptog. perscidptus. 

 Collections. — H. Lapworth, 0. T. Jones, and the Authors. 



a. Reverse aspect, but showing sicula. 



Enlargement of PI. XXXI, fig. 12 a. 



b. Obverse aspect, showing sicula and 



origin of th. I 1 . Enlargement of 

 PI. XXXI, fig. 12 c. 



c. Ibid. Enlargement of PI. XXXI, fig. 



12 b. 



Var. diminutus, var. nov. Plate XXXI, figs. 13 a — c. 



Associated with the typical Mesog. modestus there is occasionally found a form 

 resembling it very closely but differing in its smaller size and in having the 

 Fig. i82.-Mesogra P tus modestus, var. thecse more closely set. We consider it worthy of 



varietal distinction. 



The length never exceeds 15 cm., and the 

 breadth increases quickly from "6 mm. to about 1*5 

 mm. The thecas present the same peculiarities as 

 those of the typical form, but number sixteen to 

 fourteen in 10 mm. 



Horizon and Localities. — Llandovery, Birkhill 

 Shales, Skelgill Beds (zone of Diplog. vesicidosus). 



diminutus, nov. 



1 



iS 



V 



m 



Proximal end, reverse aspect. Enlarge- 

 ment of part of PI. XXXI, fig. 13 a. 



S. Scotland : Dobb's Linn. Lake District 



Skelgill, 



35 



