NORTH AMERICAN COLEOPTERA. 269 



tiot sensitive, while the iiun)ber may be still greater, reaching eight 

 in Geotrupini. In many families the greater number of antennal 

 joints have sensitive surfaces, while in others that quality is confined 

 to a limited number of antennal joints, In the first category we 

 find the Adephaga, Elateridse, Buprestidse, Cerambycid?e, Chryso- 

 melid^e, while in the second are the Scarabaeidse, the Anobiide and 

 Bostrichide Ptinidte and Hydrophilidaj. In this latter category the 

 antenna consists of two well defined parts, the sensitive apical and 

 the basal, which has no other function than that of a mechanical 

 support. 



It is evident that inasnuich as the basal portion of the antenna is 

 devoid of the structure which makes the antenna an organ of special 

 sense, the number of the joints has no importance so long as they 

 subserve their mechanical function. It is in the joints from the 

 second to the sensitive club, of Avhatever form that club may be, that 

 plasticity of structure is illustrated to the confusion of systematic 

 students. 



In the case which has given rise to the preceding discussion we 

 observe constantly two basal joints of nearly equal dimensions in all 

 the species, three terminal sensitive joints not very unlike specifically, 

 and between the two a number constituting a funiculus composed 

 of four joints in all (with three exceptions), some of which are very 

 small, and it is not remarkable that joints 4-5, the smallest of all, 

 should be found to have coalesced in the two species mentioned. For 

 the same reason the existence of a species (or possibly a genus) with 

 a still further reduction to a total of seven joints might have been 

 expected, an occurrence which Dr. Sharp says he has observed in 

 Metacymus (Biol. Cent. Am. i, 2, p. 65). As there is no reason to 

 doubt the accuracy of the latter statement, it may be safely assei-ted 

 that seven joints is the limit beneath which the number will not go 

 in Hydrophilidce. 



The following table will assist in the recognition and arrangement 

 of our species: 



Form oblong, fully twice as long as wide; tarsi slender, the posterior fully as 

 long as the tibia; pro- and mesosternum absolutely simple; posterior 

 femora not or feebly pubescent 2. 



Form elliptical convex, not much longer than wide ; tarsi stouter, the posterior 

 pair shorter than the tibiae 3. 



2. — Elytra parallel, or oblong oval ; thorax very distinctly punctate, the sides 

 bordered with pale. 

 Antennai 9-jointed dissiillilis. 



