MANUAL OF THE MOLLUSCA. 



Rhynctiospika, Hall, 1859. 



Etymology, pvy^og, a beak, and spira ; in allusion to its 

 similarity in form, to Rhynchonella, and having internal spires. 



Type, Waldheimia formosa, Hall. 



Shell somewhat similar to Rhynchonella, but usually more 

 symmetrically rounded, and with less distinct mesial sinuosities ; 

 and in these characters they resemble Waldheimia. 



Valves articulated by teeth and sockets, similar to those of 

 Nucleospira ; the crura supporting two conical spires. The 

 cardinal process of the dorsal valve is a broad emarginate plate ; 

 beak of the ventral valve largely perforated. Surface plicated 

 or striated. 



Distribution, 7 species. Silurian — Devonian. United States, 

 Russia. 



Atrypa (see p. 378). 



The internal appendages of Atrypa reticularis (see Fig. 24) 

 consist of a pair of spiral cones, placed side by side, with their 

 apices directed towards the cavity of the dorsal valve; the 

 lamellae have their origin on the socket-walls, and run parallel 

 with the inner margin of the valve. 

 " The spiral cones are connected by 

 an entire and continuous loop, which 

 is confined to the rostral part of the 

 shell. The loop arises from the posterior 

 portion of the first volutions of the 

 spires, and curves gently forward 

 and upward ; the central or elevated 

 portion is situated between and be- 

 hind the cones, and forms a more or 

 less abrupt curve, or is prolonged 

 into a point directed towards the 

 dorsal valve. The existence and form of this loop have been 

 ascertained in several different varieties of A. reticularis, as 

 well as in A. spinosa, Hall." — (Whitfield.) 

 Sub-genus, Zygospipa, Hall, 1862. 



Synonym, Stenocisrna, Conrad, 1847. Spiral cones connected 

 by an entire and continuous loop in a very similar manner to 

 that shown to exist in Atrypa reticularis ; but the loop having 

 its connection with the spiral lamellae at a point relatively more 

 distant from their origin on the hinge plate, and passing over, 

 or in front of the spires. 

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