30 PALJEONTOLOGY OF NEW YORK. 



shell. In its simpler inaniresUitions it is a testaceous deposit extending across 

 the delthyriuni from its inner margins ; as its size increases it unites the 

 dental lamelliB, 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, S. granulosus and iS. 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 coi'xistent 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 Stringothyris,* 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 Cvrtia, 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 Stringothyris 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 » more detailed accoant of the structore of this organ see the discussion of the genus Stbingo- 



THTBM. 



t Mr. Jobs Yodso. of Glasgow, has shown that the epidermal shell layer is minutely punctate in 

 Splrifrr Uneatiit (see Davidsos, 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. 



