316 



BRITISH GRAPTOIJTES. 



Eetiolites in this last respect, but the intervening test is not reticulate as in that 

 genus. 



Ketiograptus Geinitzianus, Hall. Plate XXXIV, figs. 7 a — d. 



1859. Beteograptus Geinitzianus, Hall, Pal. New York, p. 518. 



1860. Beteograptus Barrandi, Hall, New York State Cab. Nat. Hist., 13th Aim. Eeport, p. 61, 

 figure. 



1873. Clathrogrwptus cuneiformis, Lapworth, Geol. Mag., vol. x, p. 559. 

 1876. Clathrograptus cuneiformis, Lapworth, Cat. West. Scott. Foss., pi. iii, fig. 63. 

 1908. Betiograptus Geinitzianus, Ruedeinaim, Grapt. of New York, part 2, p. 463, pi. xxix, figs. 5, 6 ; 

 pi. xxxi, figs:. 9—17. 



Figs. 209 a, b, 



and c.—Retiogravtus Geinitzianus, 

 Hall. 



Polypary small, with sub-parallel margins, not exceeding 1*5 cm. in length 

 and widening rapidly to a maximum breadth of 2 mm., which is then 

 maintained. Test continuous, greatly attenuated, but locally strengthened 

 by lists into a more or less complete clathriawith sub-quadrangular meshes. 

 Sicula visible in its apertural region only. Thecae fourteen in 10 mm. 



Description. — The most striking charac- 

 teristic of this form is afforded by the 

 notable development of the thickening 

 of the salient edges and angles of the 

 polypary and thecas, and the simultaneous 

 attenuation of the intervening parts of the 

 test. The result is that the fossil gene- 

 rally presents itself upon the surface of 

 the rock in the form of a clathria of coarse 

 strand-like threads and cross-bars, in the 

 panels of which occur patches of the 

 intervening attenuated membrane of the 

 test, which also irregularly borders or 

 shades into the strands and cross-bars 

 themselves. 



The septal strands are usually well 

 defined, and one (if not both) is distinctly 

 zigzag. The lists of the parietal lattice 

 are often conspicuous and generally edged 

 by a membrane ; they have an ascending 

 direction, and are gently curved, arising 

 at a wide angle from the septal strand, 

 flowing gracefully into the apparent 

 ventral strand, the panels thus circumscribed being pentagonal or sub-elliptical 







1 



/ 





a. Complete specimen, in relief, showing sicula, both 



parietal lattices, and part of one of the ventral 

 ones. Enlargement of part of PI. XXXIV, 

 fig. 7 a. 



b. Compressed specimen showing clathria, enlarge- 



ment of part of PI. XXXIV, fig. 7 c. 



c. Specimen showing the two ventral lattices torn 



apart and twisted. Birnock Water, Glenkiln 

 Shales. Coll. Lapworth. 



