Monograph of the Crustacea of the Cincinnati Group. 139 



lobes, the longitudinal grooves continued slightly into the buckler, and 

 more distinctly into the caudal shield ; the lateral lobes are rounded at 

 their extremities, and flattened in such manner as to allow each lobe 

 to slide easily under the lobe immediately preceding. Caudal shield 

 subtriangular, convex, equaling the head in size, with the posterior 

 termination rounded ; on the central part, when decorticated, a slight 

 elevation may be traced, which has the appearance of a continuation 

 of the middle lobe. 



It varies in size from less than an inch in length to 18 inches or 

 probably more. Fragments abound throughout the blue limestone, 

 from the lowest exposure to the highest. Good specimens are rare, 

 though not so extremely rare as good specimens of the megistos. 



There is a marked difference in the proportions that the head shield 

 bears to the tail plate, and the tail plate to the abdominal sections in 

 sj)ecimens of different sizes. This variation gave rise to the species 

 Isotelus planus (De Kay), though it can be no more than a variety. 



A rolled up specimen was described Isotelus stegops (Green), though 

 from the description it does not seem to have presented any very great 

 variation from a tyj^ical si^ecimen. 



Genus Calymene — (Brong, 1822). 



Cephalic shield semi-circular, with a reflected anterior margin, and 

 obtusely rounded lateral angles ; glabella narrower in front than at base, 

 sides marked with three maxillary furrows and tubercles on each side, 

 the anterior smallest ; eyes about the middle of the cheeks, reniform, 

 prominent, strongly facetted ; eye line advancing to the anterior margin, 

 where being confluent with the marginal suture it joins that of the 

 opposite side ; over the eye it makes a small curve outward, defining 

 a semi-circular eye lobe, from whence it extends to the lateral angles, 

 which it bisects on each side ; thorax of thirteen rings, axis very convex, 

 lateral lobes wider than the axis, bent do^\n at their ends with large 

 facets, each with a strong pleural groove, angularly bent down and con- 

 fluent at its end with the posterior margin ; 'pygiclkim narrower than 

 the buckler, semi-oval, with distinct seven, nine, or eleven joints, 

 prominent axis, and broad convex, lateral lobes, the segments of which 

 are flat, about equal to the axial in number, and divided by a sulcus at 

 their ends. 



In a wider sense, by McCoy, it is described as follows : 

 Lateral angles of the head obtusely rounded, exactly bisected by the 

 facial sutures ; eyes small, "hiantf glabella narrower in front than at the 

 base; thoracic segments thirteen. 



