THE AEENIG AND LLANDEILO EOCKS OF ST. DAYId's. 649 



general line of direction of the branch. They gradually lengthen 

 as they extend along the branch, increasing their angle of inclina- 

 tion first to 45 degrees and ultimately to about 90 degrees, and their 

 length from one twentieth to one fifth of an inch. At the distal end 

 of the branch they overlap each other for more than four fifths of 

 their length. The apertural and outer margins are both concave. 

 The resultant denticle is consequently very acute, and it is frequently 

 prolonged into a slender spine. 



Log. Lower Llandeilo (lower and upper zones), Abereiddy Bay. 



DlDYMOGEAPTUS PT/ECILLATTTS, LapW., Sp. nOV. PL XXXY. figS. 



3 a-d cl. 



1851. Graptolithus geminus, Scharenberg (non His.), Ueber Grapto- 

 lithen, tab. i. figs. 1, 3 & 4. 



Branches rigid, one and a half to two inches in length, curved at 

 their commencement, but soon becoming straight, and ultimately 

 including an angle of from 330 to 345 degrees ; originally 

 narrow, but slowly widening out to a point of maximum 

 diameter and then rapidly contracting to their distal extremi- 

 ties ; sicula short, pointed ; hydrothecaB 24 or 25 to the inch, 

 inclined at an average angle of 45 degrees ; apertural margins 

 almost straight, at right angles with the normal direction of 

 the theca ; denticles inconspicuous. 



The axillary portion of the polypary in this well-marked form is 

 narrower, and the sicula more pointed than in D. Murchisoni. The 

 branches, in place of showing anything of that tendency to cross 

 each other pointed out in that species, have the distance between 

 them continually augmented, the ultimate ventral angle never being 

 less than 15 degrees. The thecse very slowly increase in length, 

 angle of inclination, and amount of overlap, till they reach a point 

 situate within the last third of the branch. Here they have a 

 length of about one-eighth of an inch, are inclined at an angle of 

 45 degrees, and overlap each other for three fourths of their length. 

 Beyond this spot they rapidly diminish in all these points, and the 

 final thecae are often very short and rudimentary. This peculiar 

 disposition of the tbecEe gives a remarkable curving outline to the 

 ventral margin, and forms perhaps the chief characteristic of this 

 species. 



Loc. Lower Llandeilo (lower zone), Abereiddy Bay. 



Genus Teteageapttjs, Salter. 



TETEAGEArTus quadeibeachiatus, Hall, sp. PI. XXXIII. figs. 

 da, 96. 

 1858. Graptolithus quadribrachiatus, Hall, Bep. Geol. Surv. Canada 



for 1857, p. 125. 

 1863. Tetragrajpsus crucialis, Salter, Quart. Journ. Geol. Soc. vol. xix. 



p. 137, fig. 86. 



