404 Linnarsson — On the Eophyton Sandstone. 



group, and B. Grenvillensis (Billings) from the Chazy limestone of 

 North America, may be considered as the type. It is an oblong 

 body, very convex and broader at one of its ends, which may be 

 called the anterior ; it is divided into two symmetrical lobes by a 

 longitudinal furrow, which in the hinder portion is narrow and 

 shallow, but increases forwards in size, so that the front ends of the 

 lobes are completely separated from each other. The width of the 

 whole fossil does not always increase regularly and continuously ; 

 the enlargement usually takes place more slowly before the middle 

 and sometimes entirely ceases. Even when the width of the whole 

 increases continuously, the lobes taper before the middle, in con- 

 sequence of the increasing breadth of the longitudinal furrow. 

 Each Ibbe has a multitude of close, rather regular narrow ribs, 

 which in the hinder portion meet in the median furrow, forming 

 an angle, the top of which is directed backward. In the middle, 

 where their direction is often suddenly altered, they are almost at 

 right angles with the median line. In the anterior portion, where 

 the ribs of the two lobes do not touch, their projections, if drawn 

 out, would form an angle with the top directed forwards. In conse- 

 quence of this change in their direction, the ribs are crowded to- 

 gether on the sides and diverge inwards. The dimensions, absolute 

 as well as relative, vary, but not to any great extent. The 

 length I have found to vary between 50 and and 80 millimetres. 

 The greatest breadth is sometimes but little less than the length, 

 but it does not usually exceed two-thirds. The height is 

 greatest in the middle, being sometimes equal to a third of the 

 length. There is no trace of an axis. Hall considers that he has 

 found such an organ in the B. bilobatus, but, as stated by Dawson, 

 the supposed stem is undoubtedly nothing but a tube of a worm. 

 Even in the Vestrogothian species such tubes are sometimes seen to 

 issue here and there between the ribs, but never in such a situation 

 as to be mistaken for branches or stems. 



The ribs are but comparatively seldom united into such convex 

 bodies as are here described. They are usually extended almost 

 horizontally ; sometimes whole slabs are covered with such systems. 

 The ribs are in this case straight or but slightly bent, more nearly 

 parallel with each other and usually almost at right angles with the 

 median lines. The opposite ribs are seldom in contact, and the two 

 halves are therefore entirely separated. Even the approximate ribs 

 of the same lobe are seldom contiguous. Sometimes transitions be- 

 tween this form and the one above described are seen. Thus, in one 

 specimen, the ribs are more crowded and meet at the median line ; 

 each lobe is slightly convex, but of the same height throughout its 

 whole length, while the typical form is highest in the middle and 

 sloping both forwards and backwards. The specimens of the hori- 

 zontally expanded form hitherto collected have a length of from 15 

 to 80 millimetres ; the number of the ribs varies according to the 

 length, but depends also in some degree upon their being more or 

 less crowded. The breadth is between 15 and 30 millimetres, but is 

 in general nearly the same in different parts of the same specimen. 



