BRITISH FOSSILS. 



Decade V. Plate VII. 



PYGASTER SEMISULCATUS. 



[Genus PYGASTER. Agassiz, 1834. (Sub-kingdom Radiata. Class Echinoder- 

 mata. Order Echinoidea. Family Cassidulidae.) Orbicular, depressed or subconic ; 

 ambulacra simple throughout ; tubercles perforate (and crenulate ?), disposed in very 

 regular series in both areas ; anus very large, superior, generally placed near to the 

 plates of the disk ; mouth decagonal, no tubercles round it.] 



Diagnosis. P. depressus, suhpentagonus, margini postico subocuto, nec 

 trimcato; tuherculis conspicuis — primariis ambulacrorum supra binis, ad 

 marginem in series 4-5 — inter ambulacrorum 18-22 — collocatis ; basi in 

 medio valde concavd : ano major i later ibus tumidis. 



Synonyms. Clypeits semisulcatus, Phillips (1829), Geol. Yorksli. 

 vol. i. p. 104. t. 3. fig. 17. (fig. two-thirds nat. size). Buckman, in Mur- 

 cliison's Geology of Chelt., 2nd edit. p. 95. (1845). Nucleolites semisulc. 

 Desmoulins (1837), Tabl. Syn., p. 362. Pygaster semis., Agassiz, Prodr. 

 Ech., p. 185. Lamarck, Hist. Nat., 2nd edit. (1840), vol. iii. p. 353. 

 Wright, Ann. and Mag. Nat. Hist. (1852), vol. ix. p. 89. Pygaster 

 umbrella, Agassiz, Ech. Suiss. (1839), tab. 13. fig. 1-3. Desor (1842), 

 Monogr. des Galerites, 3., p. 77. t. 12. fig. 4-6. Forbes, in Morris's Cata- 

 logue, 2nd edit. 88. (not Galerites umbrella, Lamarck). 



Description. — This fine Oolitic fossil is 3 inches across in either 

 direction, and \\ inch in height. The general form is subhemi- 

 spheric, and the outline orbicular or very slightly pentagonal. The 

 base is flat and deeply concave at the mouth. 



The upper surface is most generally evenly convex, more rarely 

 the sides are flattened and the shape low-pyramidal. The vertex 

 from which the five ambulacra radiate at nearly equal distances, 

 is rather behind the centre, and as the large oblong anal opening * 

 is close to the plates of the disk, which are (always ?) lost, a broad 

 keyhole-shaped cavity is left, which occupies the central and post- 

 central portions. 



* In the young state this opening is far less conspicuous, 

 [v. vii.] 5 G 



