NATURAL SCIENCES OP PHILADELPHIA. 389 



This species cannot, with perfect propriety, be arranged under any of the 

 subgenera of the great genus Gomphus, established in the Monographic des 

 Gomphines. All these subgenera, except Erpetogomphus, have the thorax yel- 

 low with black stripes, or dark with subobsolete yellow stripes, and that sub- 

 genus disagrees with our insect in the vesicle of the vertex being "divided 

 into two tubercles," in "the $ 10th abdominal segment being equal to the 

 9th," and in "the 8th and 9th segments being but little dilated." (Monogr., 

 p. 69.) The posterior femora in rupinsulensis are short, with subequal spines 

 about two-thirds millimetre long for their entire length. Its complete measure- 

 ment will be found, some pages below, at the end of the genus Gomphus. By 

 relaxing my unique specimen I have ascertained the interesting facts, that it 

 agrees with Erpetogomphus in having no tooth on the second joint of the penis, 

 and that the first genital hooklet (hamecon) is two-branched, precisely in the 

 same extraordinary manner as in Erpetogomphus cophias. (See Mon. Gomph., 

 Plate IV, Fig. 6.) 



One of two things, therefore, must necessarily be done. Either a new sub- 

 genus must be founded to receive rupinsulensis, or the old subgenus Erpeto- 

 gsmphus must be modified so as to comprehend it. Which of the two courses 

 is adopted is a matter of opinion and taste. For my own part I would suggest 

 that Erpetogomphus be modified so as to run somewhat as follows: "Last 

 abdominal segment not notably shorter than the penultimate ; abdominal 

 appendages with their branches contiguous ; the superiors about as long as or 

 a little longer than the last abdominal segment. [In Erp. designates they are 

 considerably longer.] No tooth on the second joint of the penis. Legs short. 

 Posterior legs not extending beyond the middle of the third abdominal seg- 

 ment. Thorax with the normal dark stripes more or less obsolete. Abdomen 

 with dorsal lanceolate spots, sometimes subobsolete." If a great number of 

 species should hereafter be discovered, groups founded upon variations in the 

 coloring may be established, as in the subgenus Gomphus. 



So far as can be seen, from the very brief diagnosis of a novel Mexican spe- 

 cies, Erpetogomphus boa, published by M. Selys de Longchamp, in the Addi- 

 tions an Synopsis des Gomphines (p. 11), there is considerable similarity between 

 that species and rupinsulensis. They differ, however, not only, as is to be 

 presumed, in- the subgeneric characters noted above, but in boa having the 

 tibiae entirely brown, and the inferior appendages only two-thirds the length 

 of the superiors. Moreover the abdomen of boa is proportionally much shorter, 

 being to the inferior wing as thirty-nine to thirty-five, whereas in our species 

 it is as thirty-eight to thirty-one. No true Erpetogomphus has as yet been 

 discovered north of Texas, and all the known species are American. 



Mackogomphus ? spiniceps, n. sp. 9 Pale obscure brownish. Head with 

 the occiput straight, ciliated with black hairs as long as usual ; its upper edge 

 slightly bent forwards in the middle ; the vesicle of the vertex cariniform, 

 curving backwards in an exact semicircle, the sides of which are laminiform 

 and much elevated, and the middle and the posterior extremities much de- 

 pressed, the latter not attaining the eyes ; between each of these extremities 

 and the eye is a slender acute black thorn, as long as the second joint of the 

 antennae ; antennae black ; front projecting less than is usual in Gomphus, and 

 excised less than usual, its angulation about quadrangular, and not in an 

 acute angle as in Gomphus fraternus Say, the apex of the angle not rounded 

 off; the basal half of its upper surface is pale brown, glabrous, polished ; the 

 other half yellow, opaque, with black hairs ; its anterior surface is pale brown, 

 semi-transparent, immaculate ; mandibles brown at tip ; the rest of the mouth 

 pale brown above, yellowish with long rufous hairs beneath. Prothorax 

 largely and obscurely varied with brown ; dorsum of the thorax entirely 

 brown, except a faint pale brown, much abbreviated, oblique line on each side 

 of the central carina, indicating the place where the dorsal stripe has almost 



1862.] 



