STATE GEOLOGIST. 97 



body, exclusive of the head, (which is covered with a separate 

 and similEir plate) included in a large carapace, which is open 

 below and behind, permitting the protrusion of the posterior por- 

 tion of the abdomen, and allowing currents of water to pass with- 

 in, both for respiratory purposes and to carry to the mouth parti- 

 cles of food. 

 The carapace is composed of three parts, in one" species at least, and it would 

 seem that this is the typical structure. The middle plate (in Daphnia vetula) 

 lies over the dorsal region ; the other two spring from it, flanking |it on either 

 side, and forming the bulk of the shield. We would suggest the probable simi- 

 larity of the central shield to the caudal shield of other crustaceans, and the 

 possibility that the now larger portions ought really to be considered as accessory 

 simply. 



The Daphinoidea possess from four to six pairs of foliaceous^appendages, or 

 branchial feet which do not assist in locomotion. The eye is apparently single 

 and is a very prominent organ in all the members of the tribe, it is large and 

 furnished with numerous lenses. 



The superior antennae are small, often obsolete, and except in Bosmina only 

 one or two-jointed. The inferior antennae are large, being the true organs of 

 locomotion, and several-jointed. 



The abdomen is incurved, mobile, furcate at the extremity and bears two 

 prominences on the dorsal angle which are the origin of setae. 



The carapace is often beautifully reticulated and sometimes there are more 

 than one sort or series of these markings. The Daphinoide are distinguished 

 from Cyproidea by the presence of posterior foHaceous legs, which is considered 

 by Dana as of greater importance than the more obvious pecuHarity — the ex- 

 clusion of the head from the carpace. 



Prof. Dana has united the genera of Dr. Baird's Daphniadce, Polyphemidce 

 and Jjynceidce in this tribe, and this seems appropriate, since there cannot cer- 

 tainly be as wide a gap between Daphtiia and Lynceus as between the curious 

 Bosmina and Daphnia, yet the latter two were united in one family and Lyncens 

 separated as totally distinct. The chief pecuharities which lead to the separation 

 of Lynceidse were first, the fact that they possessed in front of the eye a " black 

 spot" of unknown use, and second, that the head was produced in front to form 

 a more or less prominent beak. 



But it has since been ascertained that the black spot is a common feature 

 among the species of the whole tribe and, according to modem authors, is in 

 someway connected with the base of the superior antennae and serves, probably, 

 as an organ of hearing. 



The characters of the head seem to have been misunderstood. In all the 

 species of the Daphinoidea which I have examined, the head seems to be covered 

 with a curved plate or carapace, within which is the insertion of the organs of 

 the head. Now a little change of position under the microscope serves to give 

 to the anterior portion of this covering on acute or obtuse appearance, depend- 

 ing on which side of the carapace is in the focus of the instrument. As the 

 shape of the beak is used as a generic character in this sub- division it seems 

 quite probable that the matter will need further revision. The fact that the 

 intestine is or is not convoluted was shown by Dana to be of no generic im- 

 portance. 



The chara<jteristic8 of the tribe may be best seen as illustrated in the subdi- 

 visions. 



7 



