162 [Assembly 



aa yet received descriptive and illustrative notice, and others remain 

 as yet undescribed. 



The subject of the present paper may well be included in the list of 

 mirable forms from this region. Failing to find any publication of 

 allied species in the literature on the lamellibranchiata, I am induced 

 to present the only species known to me, and to refer it to a new genus. 



SPIEODOMUS, K. G. 



[^jTtaipa, sjjira ; dejxco, domus.] 



SpIRODOMUS INSIGNIS, 71. sp. 

 Plate 13, figs. 1-5. 



Description. — Shell cylindrical, longitudinally spiral or twisted, the 

 margins making nearly a complete volution ; length nearly three 

 times the breadth. Ventral margin entire, curved. Dorsal margin 

 of the same curvature as the ventral border. Hinge-line extremely 

 short or obsolete. 



Valves compressed posteriorly, gibbous along the middle and ante- 

 rior portipns. The transverse section of the two valves in position, in 

 the middle of their length, is broadly elliptical. The greatest depth 

 of both valves is in the anterior third. 



Beaks terminal, small and appressed, situated in the middle of the 

 anterior end. Umbo not prominent. 



Test thin. Surface marked by very fine stri^ of growth. Ventral 

 and dorsal margins strongly crenulate, produced into a thin reflexed 

 expansion. Posterior margin simple, slightly bent. 



The cast of the interior is marked by a small, distinct pit or depres- 

 sion at the anterior extremity, and just below is, apparently, a small 

 muscular scar {vide fig. 4). At this point the pallial line originates 

 and extends parallel to the ventral margin, to the lower third of the 

 posterior extremity, terminating in a small muscular scar. The upper 

 part of the posterior muscular impression, in the cast, is marked by 

 two or three nodes, from which a low rounded ridge extends anteriorly 

 about one-third the length of the shell and then becomes merged into 

 the general convexity of the valve. This ridge was probably produced 

 in the recession of the muscular scar consequent on the growth of the 

 shell. 



A specimen of medium size has a length of about 82™™; greatest 

 breadth of the cast 30™™; breadth of posterior end 21™™; greatest 

 depth of both valves as conjoined 19™™; and the width at this point 

 is 24™™. 



Having but a single species at this time, the generic characters are 

 necessarily included in the specific description. The features of the 

 most general importance are: The equivalve, elongate spiral form; 

 the terminal beaks; the absence of a proper hinge-lme, and the mus- 

 cular impressions situated at the two extremities of the valves. 



I know of no recent or fossil genus with which this shell may be 

 satisfactorily compared. A somewhat similar form could be produced. 



