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SILURIAN TRILOBITES. 



CALYMENIDtE, Bronyniart. 



The group so named must for the present be restricted to two genera, both of which 

 are the most common of all Trilobites, and grow to a large size. If we admit more than 

 Calymene and Homalonotus into the family, we shall have a heterogeneous group, some of 

 which have the facial suture ending in the posterior angles, and others on the posterior 

 margin — some with more and others with less than thirteen rings to the body. But if 

 these latter are considered to form a distinct family — the Conocephalidse, the two genera 

 above named constitute a most natural group. Calymene has the trilobation of the body 

 complete. Homalonotus is singularly deficient in this respect. Some of the species, 

 indeed, of this last genus are sufficiently trilobed to connect them with Calymene, others 

 are so slightly lobed that the term Trilobite is a misnomer as applied to them. But in 

 the essential characters of thirteen body-rings, and a notched labrum attached to a distinct 

 rostral shield, both agree; and the facial suture in each ends exactly on the angle, a position 

 midway, it will be observed, between that in the two groups already described, and that 

 which it assumes in all the ' Primordial' genera of the Conocephalidae, which have it posterior. 

 Restricted thus, we have a numerous set of species, belonging to two natural genera, 

 included in this group. They have a thick granulated crust, often ornamented and even 

 spinous superficially, but without produced spines or angles to any part of the margin. 



Calymene, Bronyniart. 



One of the most graceful and compact of all the Trilobite group ; the head and tail 

 well developed, but not extravagantly so ; the former with a three-lobed glabella, very 

 convex and narrowed in front, and with prominent supine eyes, which have evidently a 

 very thin cornea, in which, only very rarely, the lenses are visible ■} a thick margin to the 

 head, the suture being in front submarginal and subtending a broad rostral shield. This 

 bears a notched labrum, with a central gibbosity, conspicuous in all the species. Body of 

 thirteen rings, the axis convex, and the pleurae facetted and rounded at the ends. The 

 tail, of about five lateral segments duplicated throughout, and the convex axis reaching to 

 the very end in all the species, and with eight or ten rings to it. 



The range of the genus is from the Arenig Group to the Ludlow Rocks. In Bohemia 

 alone is Calymene known to rise somewhat higher ; the upper limestones of that country, 

 containing Calymene, being generally considered to be the lower beds of passage in the 

 Devonian strata. 



1 Hall, in his ' Palaeontology of New York,' has figured the lenses. I have never seen any traces of 

 them. 



