CLASS II 



BRACHIOPODA 



393 





Fir;. 585. 



A, Clitanibonites adscendens (Pand.). Ordovician ; Pawlowsk, near St. Peters- 

 burg, i/i. B, C, C. Kquamafus (Pahlen). Ordovician ; Kuckers, Esthonia. B, 

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

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



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 

 chiiidinm. Dental 

 lamellae of ventral 

 valve very strongly 

 developed, nniting 

 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 : Remipronites Pander. Biconvex Orthid-like forms with fine non-lamellose 

 striae. Ordovician ; Esthonia. 



Pohjtoechia 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. 



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 externally pitted. Ordovician. 

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



to 



Fig. 58t;. 



PmcMnbonites aequirostris {%<i'i\\ot\i.). Ordovician (Vaginatenkalk) ; 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 triangular 



