ATRYPA. 117 
2. ATRYPA DESQUAMATA, Sowerby. Pl. XIII, figs. 13—15. 
1840. SprriFER aFFINIS, Sowerby. Geol. Trans., ser. 2, vol. v, pt. 3, pl. lvii, fig. 11. 
1840. Arrypa DEsQuAMATA, Sowerby. Ibid., pl. lvi, figs. 19—22. 
1865. —_— — Davidson. Brit. Foss. Brach., vol. iii, pt. 6, p. 58, 
pl. x, figs. 9—13, and pl. xi, figs. 1—9. 
1882. — _ — Ibid., vol. v, pt. 1, p. 39, pl. i, figs. 15, 15a. 
1885. —  RETIOULARIS, var. EXPLANATA, DESQUAMATA, and PLANA, Maurer. 
Abhandl. Grossh. Hessisch. Geol. Landes., 
vol. i, pt. 2, pp. 181, 182, pl. vii, figs. 31—34. 
Localities —From Lummaton there are sixty specimens in my Collection, 
three in the Bristol Museum, eleven in the Woodwardian, and four in the 
Torquay Museum, one of which shows the beginning of a tubular frill. From 
Wolborough are numerous specimens in Mr. Vicary’s Collection, twenty in the 
Museum of Practical Geology, eighteen in the British Museum (including 
Davidson’s figured specimen, Pl. X, fig. 13, which is a very peculiar shell), and 
three in the Bristol Museum. 
Remarks.—Sowerby has accurately figured this species under the names of 
A. desquamata, var. compressa, and Spirifer affinis, but 1 am not sure whether some 
of the specimens in the Museum of the Geological Society drawn by him as types 
of A. desquamata are not really large specimens of A. reticularis. Davidson, 
however, has clearly described the form. 
Among of the chief distinctions are its flatness, its general transverseness 
(though some examples are circular), the frequent and sometimes strongly 
marked median depression in the dorsal valve, the flatness of its margins, its large 
flat wide area, its exposed foramen, its erect and elevated beak with angular sides, 
and the frequent divarication of its ribs close to the margin. These points seem 
fairly constant, and are especially noticeable in young shells. In my Collection 
is a curious small variety, in which the area is perpendicular to the dorsal valve, 
and the foramen is bounded by a high margin and separated by a very wide 
deltidium : it would, indeed, not be supposed even to be an Atrypa were it not 
united to typical shells of the present species by an unbroken chain of intermediate 
specimens. Some of the smaller specimens, in fact, are so like Orthis or Strepto- 
rhynchus that but for the characters of the beak, &c., they might readily be taken 
for such shells. Thus Sowerby has given the name of Spirifer affinis to one 
small example, while the shell in the Geological Society’s Collection, which 
Davidson figures on his pl. xix, fig. 3, as “ Strophomena wmbraculum ?= Orthis 
tenuistriata, Sowerby,” is closely mimicked by wide examples of the present species. 
As in A. reticularis, the ribs seem to increase in number with the size. 
