I 



CLASS II 



BRACHIOPODA 



393 



the greatest diameter of the shell. Cardinal area of the ventral valve high ; 



delthyrium broad, and covered by a perforate deltidium ; that of the dorsal 



valve covered by a 



chilidium. Dental 



lamellae of ventral 



valve very strongly 



developed, uniting 



to form a concave 



spatulate plate or 



spondylium. This 



plate serves for the 



attachment of 



muscles, and is sup- 



ported by a median 



septum extending 



for about one-half the length of the valve. External surface radially striated 



and often lamellose. Shell substance impunctate. Ordovician ; North Europe 



and North America. 



Subgenus : Hemi^nonites Pander. Biconvex Orthid-like forms with fine non-lamellose 

 Striae. Ordovician ; Esthonia. 



Polytoechia Hall and Clarke. Like Clitambonites but with the spondylia 

 supported by three septa, thus dividing the umbonal cavity of the ventral 

 valve into five Chambers. Ordovician ; North America. 



Scenidium Hall (Mystrophora Kayser). Small orthisinoid shells with the 

 delthyrium partially closed by a concave imperforate deltidium. Cardinal 

 process extending as a median septum throughout the length of the shell. 

 The septum is sometimes greatly elevated anteriorly in Devonian species. 

 Ordovician to Devonian ; North America, Europe and the Urals. 



Fifi. 585. 



A, Clitambonites adscendens (Fand.). Ordovician ; Pawlowsk, near St. Peters- 

 burg, i/j. B, ü, C. squamatus (Pahlen). Ordovician ; Kuckers, Esthonia. B, 

 Interior of dorsal valve, showing edge of the chilidium. C, Interior of ventral 

 valve, showing spondylium, septum and deltidium (after Pahlen). 



Family 3. Porambonitidae Davidson. 



Derived {out of Syntrophiidae), progressive, semi-rostrate Pentamerids, with the 

 deltidia and chilidia vanishing more and more in time. Spondylia and cruralia 

 present, but the former tends to thicken and unite with the ventral valve. Ordovician 

 to Lower Devonian. 



Subfamily A. Porambonitinae Gill. 

 Non-plicated Porambonitidae, with the shells ezternally pitted. 'Ordovician. 

 Porambonites Pander (Isorhynchus King) (Fig. 586). Shells similar 



A B C D E F 



to 



Fig. 380. 



Porambonites aequirostris (Schloth.). Ordovician (VaginatenRalk) ; St. Petersburg. A-C, Anterior, lateral 

 and posterior aspects of shell, Vi. D, Punctate surface, magnified. E, F, Interior of ventral and dorsal valves, 

 respectively. 



Parastrophia in form, but without plications, and with obtusely triangulär 



