88 



BRITISH GEAPTOLITES. 



showing a 



width of 



it attains a length of 



Figs. 24 a, b, and c. — Didymograptus 3Iur- 

 chisoni, Beck. 



Proximal end, impression of obverse view. 



Abereiddy Bay. Coll. G. L. Elles. 

 Proximal end, obverse view, on same slab 



as type specimen of D. Murchisoni. Gelli, 



Lliiiidriiidod. 

 Proximal end, reverse view. Enlargement 



of part of PI. III. fig. Ij. 



specimens the stipes, after running parallel for a short distance, gradually 

 approximate, until the thecal margins are in contact, and in some extreme cases the 

 stipes may actually cross each other near their distal ends. The maximum width 

 is only attained in the largest individuals; specimens 

 2-5 — 3"2 mm. are of commoner occurrence. 



The sicula is large 

 about 3 mm., and widens rapidly in the 

 direction of its aperture, which has a breadth 

 of about •75 mm. The first theca (tli. 1^) 

 originates about 1 mm. aboye the aperture of 

 the sicula, and running outward and down- 

 ward (at all events in the obverse aspect) 

 leaves the aperture free on the left side for 

 a small fraction of its length. The crossing 

 canal is wide and but slightly oblique ; o's^-ing- 

 to its width and the direction of its growth, 

 a considerable part of the apertural region 

 of the sicula is concealed in the reverse 

 aspect of the polypary. The origin of the 

 stipes is fairly symmetrical. 

 The number of tliecse in a given unit of length varies in different specimens, 

 but is usually the same in the same individual at any point along the stipe. The 

 number is normally fourteen in 10 mm., Ijut it may be as low as twelve in 10 mm., 

 or in very exceptional cases it may rise to sixteen in 10 mm. At the very 

 commencement the ventral margin of each cell is in contact for about half its 

 length, but near the distal extremities of the stipes the thecae may overlap four 

 tifths of their length. The angle of inclination of the thec^e varies in different parts 

 of the stipes from about 25° near the sicula to 45° in the maturer parts of the stipes, 

 while in very long forms the angle may become as much as 60°. The thecae are 

 at first about 1"3 mm. in length, but increase gradualh^ until they reach 5 mm. 

 as a maximum. 



He marls. — The above description has been maiuly drawn up from the type 

 specimens figured by Murchison in his ' Silurian System,' now in the museum 

 of the Geological Society of London, supplemented by details drawn from similar 

 forms in S. Shropshire. 



This form, D. Murchisoni, is most intimately connected with the form D. gemimtf< 

 of Hisinger and other Swedish authors. Owing to the kindness of Dr. Tornquist, 

 of Lund, who has furnished us with specimens of D. gemiima from the typical 

 SAvedish locality, we are enabled to discuss the question of the resemblances and 

 differences existing between these two forms in some detail. 



The type specimens figured by Sowerby in illustration of Murchison's species 



