378 



ARTHROPODA. 



while the other and much larger one is made up of the amal- 

 gamated extremities of the remaining segments. 



1 6. ENCKINUKID^E. This family is principally or exclusively 

 characteristic of the Upper and Middle Silurian periods, and 

 is related to the preceding through the intervention of Am- 

 phion. The head is fairly developed, the genal angles rounded 

 or pointed, and the facial sutures discontinuous, and cutting 

 the outer angles of the cephalic buckler. Eyes are present, 

 though not of large size, and the glabella may or may not 

 exhibit distinct lateral grooves. The surface is tuberculated, 

 and some or all of the body-rings may bear spines. The 

 thorax consists of eleven segments ; and the tail, though 

 moderate in size, has a well-marked axis, which is composed 

 of very numerous rings. 



Fig. 236. A, Head-shield of Encrinurus punctatus ; B, Head-shield of Cromus intercostatus ; 

 C, Head-shield of Cybele bellatula; D, Head-shield ofBronteus campanifer ; K, Tail of the same. 

 Silurian. (After Barrande.) 



In Encrinurus (fig. 236, A) the glabella is pyriform, strongly 

 tuberculated, with the lateral furrows almost obsolete. The 

 body-rings are eleven in number, and the axis of the pygi- 

 dium is composed of extremely numerous rings. The genus 

 is characteristic of the Middle Silurian or of the base of the 

 Upper Silurian (Llando very period). In Cromus (fig. 236, B) 



