STATE GEOLOGIST. 75 



type; moreover the flattened appendage of the last foot is like that 

 of Eurycercus. 



Even in the form of the shell there is a combination of charac- 

 ters; the anterior part of the shell has the form peculiar to Lynco- 

 daphnidas; but posteriorly it again expands and becomes truncate be- 

 hind; the form in the adult is not unlike that of some Lynceidse, 

 but the young has a long spine posteriorly exactly like the spine 

 of Daphnia. The latter fact is very instructive, for it indicates that 

 the theory proposed (Am. Naturalist, 1882, p. 815) to explain the 

 origin of this appendage is probably the correct one. Professor 

 Leuckart suggested that this spine was a balancing rod intended 

 to keep the proper equipoise over the center of gravity; but it is 

 difficult to see why these long-bodied forms, in which the greater 

 part of the weight lies "abaft" of the pivotal point — the base of the 

 antenuse — should be thus provided while the shorter forms are not. 

 We conceive that it is an apparatus for effecting the moult of the 

 inner lining of the brood cavity of long-bodied and tender-shelled 

 animals such as Daphnia and the present genus. The great develop- 

 ment of the head in the crested Daphnidse may undoubtedly be ex- 

 plained upon Prof. Leuckart's theory, 



Sp. 1. Lj'ncodapliTiia iiiacrotliroides, Herrick. 



(Perhaps =■ Ofryoxus gracilis, Sars.) 



Notes on Cladocera of Mtnn. , p. 247. 



Sab-rectangular, greatly elongated, truncate behind, with a 

 slight spine above; head and eye small, fornix moderate, beak 

 truncate; antennules rather long, slightly curved, tapering a little 

 toward the end, whence spring three lanceolate spines and several 

 sensory filaments, five stout spines behind, above the middle, and 

 several more slender ones; swimming antennae very long, terminal 

 setse smooth to the joint; labrum as in Daphnia; mandible attached 

 behind a salient angle of the front margin of the shell; no ab- 

 dominal processes; post-abdomen broad above, triangular; terminal 

 claws pectinate, furnished with one very large toothed accessory 

 spine and a smaller one; the firstfoot has a hook ; the last foot 

 consists of a large oval plate which bears posteriorly the ordinary 

 branchial coil, here shaped like a thumb and forefinger. The young 

 is of a different shape and bears a long spine. The male is unknown. 



