CAMPTOPLITES 435 



A finger-shaped organ (cf. Marcus, 1939, p. 201) is highly developed in many of these 

 avicularia (see Busk, 1884, p. 41, and my Figs. 43 D, F, J, 45 G). 



In describing avicularia, the word "head" has been used in the past both for the 

 muscle-chamber, as opposed to the rostrum or beak, and in a more general sense to 

 cover the whole head including the beak (as, for example, in Kluge's terms long-headed 

 and short-headed avicularia). I do not consider these two senses likely to lead to con- 

 fusion and have continued to use both. 



Typical bird's-head avicularia in which head and beak are more or less clearly dis- 

 tinguishable are present in C. bicornis, C. lewaldi and C. giganteus. They are of two 

 kinds. In the larger kind the lower edge of the beak is curved to form a down-turned 

 point (Figs. 43 D, F, J, 45 G). In the smaller kind the lower edge is straight (Fig. 

 43 C, K). These are the large and small long-headed avicularia of Kluge. 



The other avicularia of Camptoplites have no clear demarcation of beak from head. 

 They differ in size, in the curvature of the upper surface in profile, and in the relative 

 lengths of the planes of the opesial and mandibular surfaces, and the angle between 

 them. There are large, hook-shaped avicularia in which the two planes form an angle 

 on the under surface of considerably less than 180 (Figs. 42 B, 46 D, 47 A, B, C, E, G, 

 48 D, 49 E, F, 52 B, 55 A, C, E, G) and smaller ones in which the under surface is 

 almost or quite flat (Figs. 41 C, 42 C, 46 E, 47 D, F, 48 E, F, 51 B, 52 C, 55 B, D, 

 F, H). These two types are widely distributed in the C. reticidatas group, defined below 

 (p. 436), and are also found in C. tricornis and C. angustus. 



The round-headed avicularia (" kurzkopfige avicularien" of Kluge), found with the 

 long-headed forms in C. bicornis, C. lewaldi and C. giganteus, are only distinguished 

 from the small, flat-surfaced type, just described, by their larger and stronger build and 

 nearly circular outline. The whole head is at least as high as long (Fig. 43 A, B, G, 

 H, M, N). 



A curious point to be noticed in Kluge's figures, which is confirmed by examination 

 of the present material, is that the oblique calcareous thickening, found at the junction 

 of beak and head in the avicularia of Bugula, is usually absent in Camptoplites, but is 

 present in the long-headed avicularia, both large and small, of C. lewaldi (Fig. 45 G) 

 and C. giganteus (Fig. 43 J, K). It is also present in the small long-headed avicularia 

 of C. bicornis var. quadriavicularis (Fig. 43 C), and sometimes in the large long-headed 

 ones of that variety. 



In both C. giganteus and C. bicornis var. quadriavicularis a few large round-headed 

 avicularia have been seen at an early stage of development. The head is oval and clearly 

 marked off from the stalk, and, before any calcification appears, contains much tissue, 

 in which a cavity with narrow lumen and epithelial walls can sometimes be seen. 



The presence of two avicularia on a single zooecium was noticed in C. bicornis and 

 C. reticulatus var. unicornis by Harmer (1926, p. 452). Instances of this can be seen in 

 several of Kluge's figures, and in all the species examined by me, except C. lunatus and 

 C. atlanticus. The exceptions may perhaps be connected with the presence of only one 

 kind of avicularium in these species, a supposition supported by the fact that the two 

 avicularia on any one zooecium are usually of different kinds. Two of one kind have. 



