. The Devonian crinoids of the State of New York. Crinoidea, Fossil; Paleontology; Paleontology. 28 NEW YORK STATE MUSEUM the axillary on which the main arm-branches fork has equal shoulders and the axillary is distinguished as the main-axil. The ambulacral areas branch with the arms, and the series of ambulacrals formed are known as ambula- crals of the first order, second order, third order, etc., or as primary ambulacrals, secondary ambulacrals, and so on. For descriptive purposes an arm is viewed from the dorsal side and the right and left of the arm are the right and left of the observer.
. The Devonian crinoids of the State of New York. Crinoidea, Fossil; Paleontology; Paleontology. 28 NEW YORK STATE MUSEUM the axillary on which the main arm-branches fork has equal shoulders and the axillary is distinguished as the main-axil. The ambulacral areas branch with the arms, and the series of ambulacrals formed are known as ambula- crals of the first order, second order, third order, etc., or as primary ambulacrals, secondary ambulacrals, and so on. For descriptive purposes an arm is viewed from the dorsal side and the right and left of the arm are the right and left of the observer. Arms may be uniserial or biserial. In a uniserial arm the brachials are arranged in a single series. Simple arms are always uniserial, and here the brachials are more or less rectangular. In pinnulate arms the brachials tend to slope alternately to the right and the left. This tendency is increased, and either in the development of the individual or the race, the brachials become completely wedge-shaped. The next step is the arrangement of the brachials in two alternating rows with the smaller ends of the plates meeting midway, so as to form ^^f^^K^t ^<^S i-zQt a zigza§ suture line. This produces a biserial arm (figure 24). Pinnulars likewise may assume a zigzag biserial arrangement. The . , change from a uniserial to a biserial arm Figure 24 Evolution from uniserial through zigzag to compactly biserial starts at the tips of the arms and proceeds arms. (After Bather, 1900) . -i rrvi â ⢠, ^ *t ⢠j_ proximad. I his is true both m ontogeny and phylogeny. The development of biserial arms doubles the number of pinnules in a given length of arm, thereby aiding in the collection of food. The above conception of the origin of biserial arms is the one commonly accepted. According to A. H. Clark (1915, pp. 184, 189, 350, 352, 354) the biserial arrangement is more primitive in crinoids; the biserial arrange- ment is the Paleozoic type, while the uniserial arrangement arose
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectpaleont, bookyear1923
