86 BULLETIN 93, UNITED STATES NATIONAL MUSEUM. 



sutural edges are coarsely crenated. Basis flat, thin, permeated by pores, but the 

 pores do not generally run to the very center — they are, as usual, crossed by transverse 

 septa. Mouth: The lab rum is the most remarkable part. On each side of the cen- 

 tral notch there are generally two teeth, and on the two sides of the notch itself nine 

 or eleven smaller teeth, decreasing regularly in size downwards till they become so 

 minute as to be hardly visible even under the compoimd microscope. Thus, in the 

 two specimens closely examined, there were altogether twenty-two and twenty-six 

 teeth on the labrum. ^Mandibles with the two inferior teeth reduced to mere knobs; 

 maxilloe with the lower part of the edge bearing two large spines, and generally, but 

 not always, forming a step-formed projection. Cirri: The rami of the first pair are 

 but slightly unequal; in one specimen examined there were fifteen segments in one 

 ramus and twelve in the other; segments very protuberant in front. Second cirrus 

 with the segments only slightly protuberant; segments thirteen. Third cirrus longer 

 than the second pair, with the rami rather unequal in length. There is a tuft of 

 long spines on the basal segment of the pedii^el of this cirrus. Fourth cirrus twenty- 

 two segments. Sixth cirrus, in the same individual, thii-ty-four segments; on each 

 of these segments there are five or six puiis of spines. I may specify that the longer 

 ramus of the first cirrus of a large Rio Plata specimen had twenty-four segments. 



English examples examined (pi. 24, jQgs. 3-3&) have the spur of 

 the tergiim more rounded distally than the American, which have the 

 end of the spur obtuse or truncated, as in plate 24, figs. 5-5d. 

 The external furrow of the tergum is variable; sometimes distinctly- 

 depressed, and again quite flat, but with grooves at the sides or on 

 one side. On both sides of the Atlantic the shape varies from spread- 

 ing and quite depressed to C3dindric, higher than wide; always 

 smooth, unless vicariously bearing the sculpture of a supporting 

 shell, and always having exceptionally narrow radii, with steeply 

 sloping, smooth summits. 



Though widely distributed, this species is not common on the 

 American coast, so far as my experience goes, and it is partial to 

 somewhat brackish water. Darwin found it near Montevideo, in a 

 stream where the running water was wholly fresh at low tide. Speci- 

 mens were taken by Prof. E. B. Wilson on oyster shells in Quinni- 

 piac River, Massachusetts, ''below Grand Street Bridge, where the 

 water is very brackish at low tide." It occurs, however, in pure sea 

 water also. 



B. improvisus is closely related to B. ehurneus, but the latter dif- 

 fers by its striate scutum and pronged tergum when adult. The 

 young eburneus, before strife appear on the scutum, and before the 

 characteristic shape of the tergum has developed, is excessively 

 similar to improvisus; but in the latter the narrow radii have smootli, 

 slightly arched summits, the rami of the first cirri are nearly of the 

 same length, or at least not so conspicuously unequal as in B. ebur- 

 neus and the posterior rami have not so many spines. There is also 

 a conspicuous difference in the third cirri, the segments in B. eburneus 

 being highly protuberant and set with small teeth. White forms of 

 B. amphitrite differ from B. improvisus by their much broader radii, 



