Silurian Age of Mollusks. 77 



were once called, are turned out for themselves, they dig their points of 

 shells into something often the tail of a fish and pinch on. Then 

 the gills are developed, the foot grows and the auditory vesicles are de- 

 veloped in it, the young Anodonta drops off into the mud and sets up 

 for himself. Here is a tolerably high development. It is the highest 

 in the Lamellibranchiate class. The oyster is much lower. He has a 

 shorter head, that is, there is less of him in front of the hinge and more 

 back of it. He has but one adductor muscle instead of two, and no 

 foot, or almost none, no respiratory siphons. 



Dana reports the shell of a conocardium found in the topmost layer 

 of the Calciferous division of the Potsdam. This belongs to the upper 

 or siphon division of the Lamellibranchs, but in the salt water section of 

 the same, and though of high organization, it is not the highest. 

 There is a considerable expansion in the numbers of species and indi- 

 viduals of the Lamellibranchs in the later epochs of the Silurian age. 



There also appeared in the Lower Silurian, an Echinoderm the Cysti- 

 dean. This is the lower of two tribes of Crinoids, the other being the 

 Crinidea or Encrinites. They consisted of a stem with a bulbous head, 

 from the center of the top of which, in some cases, two or more arms 

 proceeded. This animal had a great run, taking on a great diversity 

 of details in form, and expiring in the early part of the Devonian age. 

 It is the forerunning type of the Crinids and Pentremites, and is proba- 

 bty an antecedent relative of the star fishes, some of which occur in the 

 upper part of the Lower Silurian. 



There is also the little animal in a bivalve shell, called the Ostracoid, 

 from its resemblance to the oysters, but it is a crustacean in reality. It 

 was generally very minute in size from one-fortieth to one-thirtieth of 

 an inch in diameter, but occasionally became as much as one-fourth of 

 an inch. It very closely resembles in structure the young of the tribe 

 which includes the cirripeds (Barnacles and Anatifas), which did not 

 appear till long after. There are genera of ostracoids still extant, but 

 probably not much like those of the Potsdam. But the most remark- 

 able animal of the age was the Crustacean Trilobite. He appears to con- 

 sist essentially of a worm composed of segments or somites arranged 

 one behind the other, as in the articulate worms, and diminishing in 

 diameter from front to rear. On each side, each of these segments is 

 extended, terminating in an edge or point. The effect is to produce a 

 lobe on each side of the worm-like central part. The limbs, if there 

 were an} T , were attached to the underside of the segments. The front 

 or head segment was widened like the rest, and covered with a crust, 

 and contained eyes, stationary and compound, like the articulate eye of 

 the present day. In general, each segment was covered with a strip of 

 the shell or crust, terminating in a point turned down at each flank of 



