678 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxv. 



But however far back the lobes may extend, the sinus between them 

 is always cut forward to the posterior border of the tirst thoracic seg- 

 ment, the one fused with the head. The base of this sinus, therefore, 

 marks the division line between the cephalo and the free thorax («., tig. 

 12). The inner borders of the lobes are usually some distance apart, 

 the sinus having a broad, more or less squarely truncated base. But 

 sometimes it is so narrow that the lobes approach each other closely, 

 and in alcoholic specimens may even overlap {nlge}\ alosa^, americanus). 



These inner lobe borders may be parallel {foUaceus, maculosus) or 

 ma}^ even converge posteriorly {niger, catoHtomi., americanus)^ but they 

 usually diverge somewhat sharply, carrying the tips of the lobes away 

 from the body {megalops, latlcauda, etc.). 



In its general shape or outline three types of carapace may be rec- 

 ognized, the orbicular^ in which the width equals or exceeds the length 

 {catostomi^ latus, etc.), the elliptical., in which the length is considerably 

 greater than the width {foliacem^ laticamla^ megalops., etc.), and the 

 obcardate, in which the greatest width of the carapace is nearer the 

 posterior end {americanus., purpureus^ etc.). It is a fact worthy of 

 note here by reason of its important bearing upon the phvsiological 

 function of the abdomen that species having a carapace of the first 

 type have comparatively small alxlomeus, and in general the larger the 

 carapace lobes the more restricted the a1)domen. 



In addition to the posterior sinus there is a broad, shallow lateral 

 sinus on either side opposite the ej^es. 



At the base of this sinus a V-shaped portion of the carapace is quite 

 clearly differentiated from surrounding portions by its greater thinness 

 and perfect transparency (fig. 2-1:). Both these differences are due to 

 the fact that here the dorsal and ventral surfaces of the carapace 

 approach each other until they come in actual contact and fuse together. 

 From the points of the V's a narrow groove extends backward and 

 inward on either side in a horseshoe-shaped curve, the toe of the shoe 

 crossing the median line about where the anterior edge of the first or 

 fused thoracic segment would naturally come (figs. 12 (text), 2, 6, etc.). 



This groove divides the carapace into three regions — the cephalic or 

 anterior oval region and the right and left lateral regions or aire {c. 

 and rt/., fig. 12). Each region is capable of more or less independent 

 motion, bending along the groove. 



The cephalic region varies greatly in shape with the varying shape 

 of the shield, being comparatively short and wide in those species 

 which have an orbicular shield, while it is long and narrow in those 

 whose shield is elongated. 



Through its center longitudinally run two ribs formed by a thicken- 

 ing of the cuticular chitin (v., fig. 12). These ribs start from the toe 

 of the horseshoe and run forward toward the anterior margin. They 

 first diverge slightly in a broad curve until they have passed the brain 



