_Paratypes_, a few larvæ from the type-locality. Explanation of the Figures A--Labial Lobes; E--Eye; EL--Externo-lateral Plate; G--Anal Gills; IL--Interno-lateral Plate; Lb--Labrum; M--Maxillary Palpus; P--Pronotal Breathing-horn; Pf--Præfrons; S--Spiracle. Fig. 1. Larva of _Dicranoptycha winnemana_, ventral aspect of body. Fig. 2. The same, head-capsule, ventral aspect. Fig. 3. The same, head-capsule, dorsal aspect. Fig. 4. The same, mentum, ventral aspect. Fig. 5. The same, hypopharynx, ventral aspect. Fig. 6. The same, antenna. Fig. 7. The same, mandible. Fig. 8. Larva of _Dicranoptycha winnemana_, spiracular disk, dorso-caudal aspect. Fig. 9. Larva of _D. minima_, spiracular disk, caudal aspect, the anal gills protruded. Fig. 10. Larva of _D. winnemana_, spiracular disk, lateral aspect. Fig. 11. Pupa of _D. winnemana_, lateral aspect of male. Fig. 12. The same, lateral aspect of female cauda. Fig. 13. The same, head and mouth-parts, ventral aspect. Fig. 14. The same, pronotal breathing-horn, enlarged. [Illustration] [Illustration] The Central Nervous System of Nucula and Malletia WILLIAM A. HILTON These bivalve forms are grouped among the simplest of the molloscs. It is especially from the condition in _Nucula_ as described by Pelseneer '91, that the conception of the most anterior ganglion being composed of four ganglia, has its chief support. Drew '01, who has also studied _Nucula_, believes that the lobes of the ganglion in _Nucula_ are superficial and that the four connectives coming from the ganglion may be interpreted in another way. That is, that one pair of nerves may represent an otocystic branch partly fused with the connective. This view seemed reasonable to him as Stempel '99 in _Solenyma_ found the otocystic nerves arose directly from the cerebral ganglion. The two species of this group used for study were collected at Laguna Beach. _Nucula castrensis_ Hinds, occurs abundantly at low tide under rocks. It is rather small for dissection, but very good complete series were obtained and stained in hematoxylin. _Malletia faba_ Dall, was much less abundant. Specimens were obtained from holdfasts or from dredging. Although this was a larger species, gross dissection was not very easily carried out on any of the specimens, but good series were made. The ganglia of _Nucula_ are easily studied in section. The cerebral mass seems composed of one main mass, partly divided into four subdivisions, the two central most completely fused, and the lateral quite distinct in places. The central portion might represent the cerebral ganglia and the lateral, the pleural if we take that interpretation. The pedal ganglion is made of right and left parts quite completely fused except at the margins. The pedal mass is the smallest of the three chief ganglionic areas. The visceral ganglia are quite widely separated and a little larger than the pedal mass.