PALAEONTOLOGY OF IOWA. 573 



This species is more extended and flattened on the summit than the A. brevicornis, 

 with fewer plates in the anal series, and the aperture consequently lower : at the 

 same time there is an additional simple plate at the base of each arm, thus raising 

 the walls of the body on' all sides except the posterior, which is much depressed. 

 The form of the base differs, and the plates of the body are less elevated, the lines 

 of suture being less deeply marked : the apicial tubercle is also much shorter. 



The three associated species, A. brevicornis, superlatus and minor, diverging from 

 the ordinary forms of ACTINOCRINUS, approach the MEGISTOCRINUS of OWEN and 

 SHUMARD in form, though the body is less extended. The A. brevicornis shows the 

 first step in the progress of development in the radial series, having one simple super- 

 radial plate; while the development of the series is carried to its greatest extent in 

 Megistocrinus evansi, and in M. lotus ( Plate I, fig. 1 of this Report). 



The same departure from the typical form of ACTINOCRINUS, so far as regards the 

 number of plates in the radial series, is as clearly shown in some of the succeeding 

 forms for which no separation has ever been proposed ; and when we institute a 

 careful comparison between these and MEGISTOCRINUS, the difference will be found 

 to lie in the broad rotund or depressed spheroidal form and smooth plates of the 

 latter, rather than in any differences of fundamental structure. 



6 a. 6 



- -. -LtJ X^,_ 



i^i C^T-I ^-?fP 8 ; -5^. The accompanying diagram illustrates the structure of 

 iTP ^(I)5\(z) iS'? 8 ^e anterior and antero-lateral rays, with the brachial 

 &&/T) v^x^Ty^ ^ and lower arm-plates. 



Geological formation and locality. In the Burlington limestone : Bur- 

 lington, Iowa. 



Actinocriims minor ( n. s.). 



BODY small, urnshaped; summit slightly convex; base 

 truncate. Basal plates concave towards the centre, with the 

 edges projecting ; line of suture between the base and first 

 radial plates deeply channelled. First radial plates promi- 

 nent, wider than long. Second radials hexagonal, about as 

 wide as long. Third radials pentagonal, supporting on each 

 of the upper oblique sides a simple brachial plate, upon 

 which rest the second brachials, and upon these the arm- 

 plates in double series. Interradial plates in series of one, 

 two, two, and two, with a little irregularity, the last series 



