- Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimorpha (or Selachii) and are the sister group to the Batoidea (rays and kin). Some sources extend the term “shark” as an informal category including extinct members of Chondrichthyes (cartilaginous fish) with a shark-like morphology, such as hybodonts. Shark-like chondrichthyans such as Cladoselache and Doliodus first appeared in the Devonian Period (419–359 million years), though some fossilized chondrichthyan-like scales are as old as the Late Ordovician (458–444 million years ago). The earliest confirmed modern sharks (selachimorphs) are known from the Early Jurassic around 200 million years ago, with the oldest known member being Agaleus, though records of true sharks may extend back as far as the Permian.
- Delta Persei (stars.astro.illinois.edu)
DELTA PER (Delta Persei). Dominated as it is by its two brightest stars, Mirfak and Algol (Alpha and Beta Persei), the next four stars of glorious Perseus, the mythical rescuer of Andromeda, are often ignored. This quartet, which consists of Gamma, Delta, Epsilon, and Zeta, are all nearly the same mid-third magnitude brightness, and add great sparkle to the northern constellation.