BRACHIOPODA. 55 



Pedicle-valve greatly elevated ; umbo arched and incurved ; with a narrow 

 median groove which becomes fainter or disappears towards the anterior mar- 

 gin. Cardinal area well defined and arched; divided medially by an open 

 delthyrium whose lateral margins bear incomplete deltidial plates. Teeth prom- 

 inent, erect, strongly recurved at the tips ; not supported by dental plates. 

 Muscular area quite restricted, consisting of narrow, elongate diductors, enclosing 

 an almost linear adductor. The entire area is sometimes divided by a faint 

 median ridge. The interior surface about the muscular area is strongly pitted. 



Brachial valve convex at the beak, becoming depressed over the pallial re- 

 gion and reflexed near the margin. Cardinal area comparatively broad and 

 standing at nearly right angles to the area of the opposite valve. Delthyrium 

 open, the deltidial covering* attaining the same degree of development as in the 

 pedicle-valve. Cardinal process narrow and much elongated, resting on the 

 bottom of the valve except at its posterior extremity which is simply bifurcated. 

 Crural plates erect, parallel ; taking their origin in the deltidial plates and ex- 

 tending about one-fourth the distance across the valve. The spirals are 

 attached by long crura, the ribbon making a few volutions only, thus forming 

 loose coils, directed laterally. The loop has apparently the same incipient 

 condition of development as in Spirifer. According to (Ehlert,! the spiral 

 ribbon bears spinules on its outer margins. Muscular impressions anterior and 

 composed of four well defined adductor scars. 



Surface smooth or with fine concentric striae crossed by indistinct radiating 

 lines ; rarely spinous. Shell substance fibrous, impunctate. 



Type, Orthis umbonata, Conrad. Hamilton group. 



Observations. — The external characters of the Devonian forms of Amboc(elia 

 ally it to Martinia, and there is little doubt that the finely punctured epidermal 

 layer is common to both. The structure of the interior, however, is so unlike 

 that of Spirifer, in its elongate and simple cardinal process, long, parallel, erect 

 crural plates and anterior muscular scars in the brachial valve, that the generic 

 value of the group is beyond question. The type of structure is essentially 



 CMlidium, Beechep. 



t Bulletin de la Society d'Etudes Scientifiques d' Angers, 1887 ; Brachiopodes du Devonien de I'Ouest de 

 la France, p. 6, pl. v, Hg. 12. 



