. The Biological bulletin. Biology; Zoology; Marine biology. 52 LEONARD P. SAYLES AND S. G. HERSHKOWITZ separately: trunk and tail regions of the body proper, as contrasted with head; fins, including spine pockets and, in male, claspers; head; mouth opening and labial pouches; ampullae of Lorenzini; olfactory sacs; eye- lids ; spiracles; external gill slits; internal gill slits; mouth cavity and pharynx. The placoid scale of Squalus acanthias consists of a basal plate and spine (as noted by Hertwig and many others). The basal plate (except in the case of stomodeals and combs) is quadrangular w
. The Biological bulletin. Biology; Zoology; Marine biology. 52 LEONARD P. SAYLES AND S. G. HERSHKOWITZ separately: trunk and tail regions of the body proper, as contrasted with head; fins, including spine pockets and, in male, claspers; head; mouth opening and labial pouches; ampullae of Lorenzini; olfactory sacs; eye- lids ; spiracles; external gill slits; internal gill slits; mouth cavity and pharynx. The placoid scale of Squalus acanthias consists of a basal plate and spine (as noted by Hertwig and many others). The basal plate (except in the case of stomodeals and combs) is quadrangular with two angles extending laterally, one anteriorly and a fourth posteriorly. The sto- modeal basal plate also usually bears four extensions, but the lateral ones, as well as the anterior, extend anteriorly. The basal plate of the comb type is similar to that of stomodeals but the angles are often indefinite and the whole basal plate frequently lacks the regularity usu- ally found in this part of other scales. The spine may be interpreted as consisting of two main elements:. Fig. 1. Left lateral aspect of a dorsal body type scale. X 75. Fig. 2. Right lateral aspect of a ventral body type scale. X 75. Fig. 3. Postero-lateral aspect of a transitional type scale. X 75. Fig. 4. Postero-lateral aspect of a body tricuspid scale. X 75. (1) a longitudinal plate which extends, in plan, between anterior and posterior angles of the basal plate; and (2) a transverse plate which extends between the lateral angles of the basal plate. The transverse component is ordinarily tilted posteriorly and rests on the posterior part of the longitudinal component. The anterior part of the latter extends onto the transverse element at least to some extent. Scale types are associated with more or less marked variations in the development and shapes of these spine components. It is well known that an opening on the under side of the basal plate connects with a system of dentinal canals which extend even int
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