172 Trans. Acad. Set. of Si. Louis. 



have the following arm formula*: 3+3+2+2+2. Of 

 the twenty-six specimens figured, ten have twelve arms, 

 the formula in Figs. 4, 7, 14, 16, 20, 21 and 24 being 

 3+3+2+2+2; in Fig. 25, 3+3+li + 2+2 ; in Fig. 22, 

 lil+3+li+li+2 ; and in Fig. 2, 3+4+2+1+2. One, No. 

 12, has thirteen arms of this formula: 3+3+2+3+2; No. 10 

 has seventeen arms arranged thus: 4+4+3+3+3. Six have 

 eleven arms, the formula in Figs. 13, 15, 17 and 26 being 

 2+3+2+2+2; in Fig. 6, 2+3+2+U + 2; and in Fig. 8, 

 3+2+2+2+2. Of the rest six have ten arms and two nine 

 arms. In Figs. 5, 9, 11 and 18 the formula is 2+2+2+2+2 ; 

 in Fig. 3, 2+li+li+li + 2; in Fig. 1, 3+2+2+2+1. For 

 Fig. 19 the formula is 3+2+2+1+1, and for Fig. 23 it is 

 2+3+2+2. 



A lack of proper regard for this occurrence has led many 

 writers to create new species, instead of considering their 

 specimens abnormal developments of existing species. From 

 the drawings it will be seen of how little value the number 

 and distribution of the arms can be for specific separation. 

 Many of Miller's species rest entirely upon the number of 

 arms. His A. Indianensis and A. Gorbyi agree in every 

 essential point with the description of A. sjilendens, the only 

 distinction being that A. Gorbyi has thirteen arms, while the 

 other two have each twelve arms.f A. Blairi is described as 

 a distinct species because it has nine instead of ten arms, 

 which is caused by one of the arms of the anterior ray remain- 

 ing undeveloped 4 The only claim A. profundus has to recog- 

 nition as distinct species is the fact that it has fourteen arms, 

 which the author thinks of specific importance. A. tugurium is 

 considered new because it has twelve arms ; and A. avoula with 

 ten arms,§ A. Iowensis with fifteen arms and A. KeoJcukensis || 

 with sixteen arms are classed as new species for that reason 



* In counting the arms I begin at the left posterior ray (ventral side 

 up) and count toward the right. Smaller figures denote smaller arms. 



t Rep. Geol. Surv. Ind. 16: 340. pi. IV. f. 1, 2. —Rep. Geol. Surv. Ind. 

 17 : 663. pi. VIII. f. 5; 664. pi. VIII. f. 9. 



% Rep. Geol. Surv. Ind. 18: 275. pi. III. f. 12-14. 



§ Bull. 111. State Mus. Nat. Hist. 6: 26. pi. III. f. 1-3; 28. pi. III. f. 

 4-6; 30. pZ. III. f. 7-8. 



|| Bull. 111. State Mus. Nat. Hist. 12: 5. pi. I. f. 1-3; 7. pi. I. f. 4-6. 



