30 
PALEONTOLOGY OF NEW YORK. 
shell. In its simpler manifestations it is a testaceous deposit extending across 
the delthyrium from its inner margins; as its size increases it unites the 
dental lamellae, fills the rostral cavity of the valve and extends forward along 
the bottom of the shell between the posterior extremities of the diductor 
muscular bands. This is its condition as usually seen in the middle Devonian 
species, 8. granulosus and S. audaculus. 
Not infrequently this plate is less thickened and extends downward with a 
convex outer surface for two-thirds the length of the delthyrium, but this 
particular form of development occurs less often in the early species. 
In all its phases it may be coexistent with the true deltidium, though the 
latter is rarely retained in growth-stages where the apical callosity is well de¬ 
veloped. From the last mentioned condition to the fully developed, adherent 
split tube of Syringothyris,^ it is but a few short steps, but these are still 
wanting among the American Spirifers, as far as our observations have gone. 
An important intermediate stage is furnished by the peculiar species which we 
have provisionally placed in the so-called Cyrtia, namely, Spirifer alius. Hall, 
an extravagant representative of the European /S. simplex. Here the transverse 
plate is thickened on its inner surface by the development of a vertical median 
ridge. In Syringothyris it is evident that the tube has been formed by the 
lateral expansion of this ridge, its margins becoming free and developing a 
tendency to incurve or curl toward each other over the median line, actually 
uniting at times while adherent to the plate, but remaining disconnected after 
the tube becomes free. 
It is very probable that the thin epidermal layer of the shell in the gran- 
ulous species of the Ostiolati was punctated; indeed the tuberculated surface 
itself, may be construed as evidence of such slight punctation.f In Syringo¬ 
thyris the shell is decidedly but variably punctated, the tubules sometimes 
penetrating the entire thickness of the shell, sometimes traversing only a 
* For a more detailed account of the structure of this organ see the discussion of the genus Syringo¬ 
thyris. 
t Mr. John Young, of Glasgow, has shown that the epidermal shell layer is minutely punctate in 
Spinfer lineatus (see Dayidson, Supplement to Carboniferous Brachiopoda, p. 275, pi. xxxiv, fig. 9), and it 
is not unlikely that the existence of a very tenuous external punctated layer will be found more generally 
prevalent among Spirifers than is now generally supposed. 
