DIELASMELLA 279 



Remarks. The original description of this species is short and unac- 

 companied by illustrations, and the type specimen is believed to be lost, 

 but the definition agrees so closely with the shell here illustrated that the 

 identification is believed to be beyond question. 



In size and convexity of the valves this species resembles G. indianensis, 

 but it differs from that species in its somewhat more pentagonal outline, 

 with the greatest width nearer the mid-length of the shell, and especially 

 in the deeper, longer, and altogether more conspicuous mesial sinus of the 

 pedicle valve and the short, broad sinus of the brachial valve with its dis- 

 tinct mesial fold or plication. This trilobation of the anterior margin of 

 the brachial valve is sometimes faintly suggested in examples of G. indian- 

 ensis, but it is never a conspicuous feature as in this species, The species 

 differs from G. turgida, with which it has sometimes been identified, in its 

 more elongate form and the less gibbosity of the valves, the trilobation of 

 the anterior portion of the brachial valve is also a much more conspicuous 

 feature, although that character is not infrequently present in G. turgida. 

 The shell described as Dielasma turgida var. elongata Weller, is apparently 

 a member of this species, the type specimen is an internal cast in which 

 the median septum of the brachial valve is clearly seen, it is proportion- 

 ally a little narrower and consequently more slender than is usual among 

 typical representatives of G. brevilobata, but it possesses the long median 

 sinus of the pedicle valve and the trilobation of the front of the brachial 

 valve, although the trilobation is somewhat obscured by a slight distortion 

 of the shell. 



Horizon. Chester group. 



Genus DIELASMELLA Weller 



Description. Shell terebratuliform, compressed. Pedicle valve with 

 well developed dental lamellae of moderate length. Brachial valve with- 

 out median septum or true hinge-plate, the socket plates well developed, 

 retreating from the lateral margins of the valve anteriorly and becoming 

 differentiated into two portions, a basal portion which joins the inner 



FIG. 34. A series of six cross-sections of the rostral portion of the brachial valve of 

 Dielasmella compressa (X 2 1 /), showing the development of the crura from 

 the inner walls of the hinge sockets, and the narrow band joining the bases 

 of the crura just before, and at the point, where they become free. 



surface of the valve and is directed obliquely inward, and a distal portion 

 which is abruptly bent in a subgeniculate angle so as to be directed 

 obliquely outward, the portion included in the angular bend of the two 

 plates is produced anteriorly into the bases of the crura, and just before 



