Mr. W. N. Lockiugton on American Alphei. 465 or less irregular in form 5 the outer edge is either straightish, slightly convex, or undulating : side arm-plates in the form of narrow ridges : arm-spines very short, covered with skin at the base, naked and minutely prickly at the tips ; four in number, except near the disk, where a few plates are armed with five; the three upper ones are subequal, and only a little more than a millimetre in length, the lowermost one very minute, and situated in a line with the tentacle-pores ; and a few of them towards the end of the rays are armed with a minute hook or double hook at the extremity. No tentacle-scale. Genital rimte furnished with a distinct irregular elon-gate-oval plate at the outer extremity, extending a little beyond the actual opening into the disk. The general colour of the disk above is purplish brown, somewhat paler on the keels of the radial shields, considerably paler beneath ; arms light brown above and whitish below ; the oral and aboral shields are of the same tint as the lower surface of the disk ; the skin covering the arm-spines is purplish brown, giving the arms a bordered aspect. Diameter of disk 25 millims., length of arm about 80, length of radial shields 10. LIII. — Remarks on some new Alphei, with a Synopsis of the North-American Species. By W. N. LOCKINGTON, The North-American species of the old genus Alpheus are now known to be very numerous, as many as sixteen having been found upon the Pacific coast, from Panama northwards, making a total of eighteen species from both coasts. Some kind of subdivision, in so numerous a group, is neces-sary for the sake of convenience ; and the presence or absence of a rostrum and of ocular spines probably furnish characters as reliable as any. Dana, in 1852, availed himself of the absence of a rostrum, and the inversion of the hands, to separate the genus Betoius, which has been generally acknowledged until Kingsley, in his synopsis of North-American species of the genus Alpheus (Bull. U.S. Geol. & Geogr. Surv. vol. iv. no. 1, p. 189), pro-posed, on what appear to me to be insufficient grounds, to reunite them. In a large series of A. minus, Say, that author found many which wanted the rostrum ; while in some other Alphei the dactylus works obliquely or horizontally, showing an approach to the characters of Betceus.