Nezv Species of Devonian Fossils. 139' 



Surface marked by three or four strong narrow plications on 

 each side of the prominent fold and sinus, which are crossed by 

 strong concentric imbricating lamellae as in D. perlamellosa. 

 The distinctive features are internal : the strong dental lamellae 

 of the ventral valve converging below, uniting with the sides of 

 the strong median septum just before reaching the bottom of the 

 shell ; the septum continuing upward into the V-shaped structure 

 thus formed by the dental lamellae. 



Dimensions of an average specimen: Length, 18 mm.; width, 

 23 mm.; depth, 13 mm. 



Discnssio>}: The Cyr//;;fl-like structure formed by the dental 

 lamelhe is a case of parallelism and does not indicate a relation 

 to Cyrtina. The new species is closely related to D. perlamel- 

 losa, but on the average it is nuich smaller and less alate, while 

 the convergence of the dental lamellae and their fusion with the 

 strong median septum are distinctive features. 



Occurrence: Rockhouse shale, at the sulphur spring and at 

 Rockhouse, both on Horse Creek, Hardin Countv. 



Delthyris octocostata tennesseensis, n. mut. 

 Plate IV, figs. 8, 9 



Description: This common Spirifcr of the Linden group in 

 western Tennessee very closely resembles the earlier D. octo- 

 costata of the Keyser formation of Maryland. It differs from 

 the latter in being proportionately shorter and more transverse, 

 the ratio of length to breadth being 3 to 5 as compared with 4 

 to 5 in the Maryland form. There is also a tendency to have 

 more plications which diverge less rapidly, the mutation fre- 

 quently developing eight and occasionally ten, whereas the spe- 

 cies usually has six and rarely eight. The ventral beak is more 

 erect, the cardinal area is proportionately larger, more clearly 

 defined, less curved, and more nearly in the plane of the valve. 

 The deltidium is wider. The dorsal valve is proportionately 

 smaller and its beak less prominent. 



Dimensio)is: Length, i6mm. ; width, 25 mm.; depth, 16 mm. 



Occurrence: Common in the Birdsong shale and rare in the 

 Ross limestone at Grandview, Perryville, Big Lick Creek, Bird- 

 song Creek, etc. 



