Very First Fossil Find. Thoughts?

[Jlarkins18]

Hi. This is my very first fossil find.
There are dozens of small fossils/shells in an unknown matrix which is roughly 20lb.
Most of the fossils look the same except for two. I added additional close ups of them to help with identification. Location of discovery is the southern california San Joaquin/santa ana dry River bed. Thanks.

 

[NJearthman]

Some thoughts on the environment of deposition of your rock specimen. The imbedded marine fossils in your rock likely did not die in place but represent an assemblage brought together after death by water currents or waves (Thanatocoenosis), then later cemented in that rock matrix.