MUSEUM OF COMPARATIVE ZOOLOGY. 235 



When dredged, the washed contents of the trawl may present several 

 bodies looking like wads of fine flax soaked in mud, and having various dead 

 shells or worm-tubes entangled therein. In this unpromising nidus is hidden 

 our gem of the sea. Long continued gentle washing under a stream from the 

 wat«r-cock finally removes most of the mud. Immersed in water, we see that 

 the nest is composed of the finest and most silky threads, inextricably inter- 

 laced and of great strength. Among them the young nestle until they are 

 ready to spin for themselves. Many of the threads centre at and are con- 

 nected with the byssal sinus, from which much force is necessary to detach 

 them. 



It will be seen from the notes on the soft parts that this mollusk is most 

 nearly related to Modiola, and not to Modiolaria, as before examination I sus- 

 pected. I have compared it with the chief types, and there is no doubt of this. 

 If we separate the polished species from the bearded mussels, this species, ac- 

 cording to Fischer, may be referred to Amygdalum Megerle (1811), from which 

 it hardly differs. Monterosato proposed the name Modiella for it ; but this 

 had been used a year earlier by James Hall (1883) for a different group. 



Modiola opifex Sat. 



Modida opt/ex Say, Journ. Acad. Nat. Sci. Phil., IV. p. 369, pi. xix. figs. 2, 2a, 2 b, 

 1825; Phil. Abbild. und Beschr. n. Conchyl., III. Modiola, p. 21, t. ii. fig. 7. 



One valve was dredged from 640 fms. in Yucatan Strait, a depth which it 

 doubtless reached in some accidental manner. This species was described by 

 Say as attached to Fecten nodosus, and found in a mass of sand grains of its 

 own collecting. Kroyer had it from Brazil, and the U. S. Fish Commission 

 has dredged it to within a few miles of Cape Hatteras, but only as separated 

 valves. It forms a transition, conchologically, between Modiolaria and the 

 group typified by Modiola semen, sometimes called Botula. 



Genus CRENELiLiA Brown. 



Crenella decussata Montagu. 



Crenella decussata Montagu, Dall, Bull. M. C. Z., IX. p. 116. 

 Nuculocardia divaricata D'Orbigny, II. p. 311, pi. xxvii. figs. 56-59, 1845. 



Habitat. Barbados, 100 fms. (Alaska, California, New England, British 

 seas, Norway, etc.). 



This little shell is proportionately a little more solid and strong than north- 

 em specimens, and the crenulations which exist in both, and from which the 

 group takes its name, partake of this difference. I have seen nothing, how- 

 ever, in the few specimens I have been able to examine, which would authorize 

 the separation of the southern form from the northern one. 



