Development of Tentacles in Hydra. 251 Halosaurns rostratus, B. 9. D. 10. V. 9-10. L. transv. 13/6. The length of the head exceeds much the height of the body. The snout very much produced, spatulate, its prajoral portion being more than one half its length. Eye of moderate size, its length being one third of the postocular portion of the head, and considerably less, than the width of the interorbital space. Maxillary scarcely reaching the front margin of the eye. The length of the head equals its distance from the root of the ventral, which is nearly entirely situated before the dorsal. Nearly all the scales are lost : but some of the lateral line remain ; they are much larger than the other scales ; and on the tail, where the lateral line approaches the loAver profile, these larger scales fill up all the space between the lateral line and the anal fin. Mid Atlantic, 2750 fathoms. NemicJithys infans. Body much less elongate and eye much smaller than in N. scolopacea. Vent twice as distant from the root of the pectorals as is the latter from the eve. Mid Atlantic, 2500 fathoms. Cyema, g. n. Mur^enid. This genus is the type of a new group of Murtenidae allied to the Nemichthyina. It combines the form of the snout of a Nemichthys with the soft short body of a Leptocephalus] but the gill-openings are very narrow and close together on the abdominal surface. Vent in about the middle of the length of the body ; vertical fin well developed, confined to and sur-rounding the tail. Pectoral fins well developed. Eye very small. Cyema atrum. The cleft of the mouth extends backwards to the end of the head. Black. Pacific and Antarctic, 1500 and 1800 fathoms. XXIX. — On the Mode of Development of the Tentacles in the Genus Hydra. By M. C. Mereschkowsky. [Plate Xn.] In my article on the new Hydroid Monohrachium parasitum* * Ann. k Mag. Nat. Hist. ser. 4, vol. xx. p. 220. 17->^
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