586 Forty-fifth Report on the State Museum. 



and punctated by coarse tubules apparently arranged in radiating 

 rows. 



(Lower Silurian — Lower Devonian.) 



Dalmanella, Hall. 1892. 



Group of Oethis testudinaria, Dalman. 



(Plate 11, Figs. 4-17.) 



Shells plano-convex or subequally biconvex. Pedicle-valve 

 usually the deeper, often gibbous, elevated at the umbo and 

 arched over the cardinal area. Hinge-line generally shorter than 

 the greatest width of the shell. In many of the species there is 

 a more or less conspicuous, undefined median fold and sinus on 

 the pedicle and brachial valves respectively. Surface covered 

 with fine, rounded bifurcating striae. 



In the pedicle-valve the teeth are quite prominent, thickened 

 at their extremities and supported by lamellae which are pro- 

 duced forward, circumscribing a rather short suboval or sub- 

 qnadrate muscular area, which is more or less distinctly defined 

 in different species and in different conditions of the shell. 



In the brachial valve the cardinal process extends forward to 

 the bases of the crural plates, where it is broadened and continued 

 thence as a median ridge separating the muscular impressions. 

 The inner surface of this process is divided by a faint median 

 furrow which produces two lobes at the posterior extremity, and 

 each of these lobes is again divided, making the process quadri- 

 lobate. Sometimes the inner divisions of the two main lobes have 

 coalesced, producing a strong median lobe and thus making the 

 process appear trilobate. In some species, at maturity, and in 

 others from abnormal growth, this process becomes a broad plug, 

 which fills the entire delthyrial opening. The dental sockets are 

 small, the crural plates often greatly elevated, especially in the 

 plano-convex forms, and they are not usually produced into a 

 ridge about the muscular area, but end abruptly. Muscular 

 impressions quadruplicate, sometimes with radiating ridges 

 extending from the lateral and anterior margins. 



Shell-substance finely fibrous and punctate. 



(Lower Silurian — Upper Silurian.) 



13S 



