Raymond — Common Devonian JBrachiopods. 297 



In an early neanic stage the hinge width is the greatest 

 width of the shell, the cardinal extremities soon become acumi- 

 nate, and nearly all adolescent shells more than 5 or 6 mm wide 

 are strongly mucronate. (Compare the first three shells of the 

 series on Plate XYI with the older ones ; also observe the 

 growth lines on specimen No. 3, Row 2.) 



It is interesting to note that in their early neanic stages these 

 transversely elongated Devonian Spirifers pass through forms 

 which correspond to the adult condition of certain Niagara 

 species. The adult of Spirifer crispus, with a fold and 8 

 plications on the dorsal valve, and a hinge width nearly equal 

 to the width below, corresponds very closely in these particulars, 

 and in its index, with a specimen of Spirifer mucronatus 

 about 2 mm long, and, except in the number of plications, with 

 the specimen of Delthyris consobrinus l mm long. 



Spirifer radiatus, with no plications, the width only one- 

 seventh greater than the length, and the width at the hinge 

 less than the width below, corresponds to a still earlier stage 

 in the development of the present species. 



Summary. 



The foregoing descriptions show that the general deductions 

 which have been previously drawn as to the character of the 

 nepionic shell, the development of the pedicle tube and the 

 deltidial plates, and the acquirement of surface characters, hold 

 good in the families here studied for the first time. Other 

 general facts will be noted under the families. 



Centronellidce. — The shape of Trigeria lepida in the nepi- 

 onic stage is almost exactly that of adult Centronella, and thus 

 another bit of evidence is added to that afforded by the loop, 

 showing its relation to the Centronellidae rather than to the Tere- 

 bratulidse. Going back to the very earliest stage, before the 

 development of the dorsal sinus, the shell has characters com- 

 mon to the superfamily, that is, a biconvex shell with the ven- 

 tral beak extended beyond that of the dorsal valve. 



Terebratulidm. — Eunella, in its early stages, is a rather sim- 

 ple, generalized type of shell, not differing greatly from the 

 very youngest stage of Trigeria, but the development of its 

 loop shows progress beyond the centronelliform stage. The 

 position of the first three punctse, which is the same as that in 

 the recent genus Terebratxdina, is interesting. 



Terebratellidoe. — The evidence that Tropidoleptus belongs 

 to this family has not been strengthened or diminished by the 

 present studies. It still rests on the form of the loop as de- 

 scribed by Hall, and later verified by Hall and Clarke. The 

 development is similar to that of the Strophomenidse, and the 



